feat(kernel): Add RB-tree/kfifo/hashtable, wait queues, futex, tiny RCU, page cache and KTest harness

Expands the kernel-and-TTY core with the primitives the higher layers have
been missing, plus an in-kernel harness that proves they work.

Data structures (lib/):
  - rbtree.c: Linux-compatible red-black tree with parent pointers, full
    insert/erase rebalancing, in-order iteration and an invariant validator.
  - kfifo.c: power-of-two byte ring with free-running indices, so the wrap
    path costs one mask instead of a branch per byte.
  - hashtable.c: separately-chained table with FNV-1a hashing, string or
    u64 keys, and rehash-on-growth at a 3/4 load factor.

Kernel subsystems:
  - kernel/wait.c: wait queues and one-shot completion barriers, backed by a
    static sleeper pool so a blocking path never re-enters the allocator.
  - kernel/futex.c: 32-bucket futex hash with value re-check, requeue, and an
    fmutex_t sleeping mutex built on the uncontended-fastpath protocol.
  - kernel/rcu.c: tiny RCU with reader nesting, deferred callbacks, kfree_rcu
    and rcu_assign_pointer/rcu_dereference publish barriers.
  - mm/page_cache.c: write-back block page cache, 4 KB pages indexed by
    (device, page), CLOCK second-chance eviction and dirty write-back.
  - kernel/ktest.c: KUnit-style harness with 6 built-in suites, 21 cases and
    735 assertions covering every subsystem above (all passing under QEMU).

LazyBox userland:
  - coreutils.c: sort, uniq, cut, tr, rev, tac, nl, seq, diff, xxd, du,
    factor, sum and truncate, reading through the VFS and handling the
    zero-length synthetic nodes that procfs and sysfs expose.
  - ktest, rcuinfo, futexinfo, waitinfo and pagecache diagnostics applets.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01L5yKJT4Wo2cV3rfPrwY5MP

lib/rbtree.c

@@ -0,0 +1,309 @@
+// Red-Black Tree implementation for SUB-OS (Linux-compatible semantics)
+#include <lib/rbtree.h>
+
+static void rb_rotate_left(struct rb_node* node, struct rb_root* root) {
+    struct rb_node* right  = node->rb_right;
+    struct rb_node* parent = node->rb_parent;
+
+    node->rb_right = right->rb_left;
+    if (right->rb_left) right->rb_left->rb_parent = node;
+
+    right->rb_left   = node;
+    right->rb_parent = parent;
+
+    if (parent) {
+        if (node == parent->rb_left) parent->rb_left = right;
+        else                         parent->rb_right = right;
+    } else {
+        root->rb_node = right;
+    }
+    node->rb_parent = right;
+}
+
+static void rb_rotate_right(struct rb_node* node, struct rb_root* root) {
+    struct rb_node* left   = node->rb_left;
+    struct rb_node* parent = node->rb_parent;
+
+    node->rb_left = left->rb_right;
+    if (left->rb_right) left->rb_right->rb_parent = node;
+
+    left->rb_right  = node;
+    left->rb_parent = parent;
+
+    if (parent) {
+        if (node == parent->rb_right) parent->rb_right = left;
+        else                          parent->rb_left = left;
+    } else {
+        root->rb_node = left;
+    }
+    node->rb_parent = left;
+}
+
+void rb_insert_color(struct rb_node* node, struct rb_root* root) {
+    struct rb_node *parent, *gparent;
+
+    while ((parent = node->rb_parent) && parent->rb_color == RB_RED) {
+        gparent = parent->rb_parent;
+        if (!gparent) break;
+
+        if (parent == gparent->rb_left) {
+            struct rb_node* uncle = gparent->rb_right;
+            if (uncle && uncle->rb_color == RB_RED) {
+                uncle->rb_color   = RB_BLACK;
+                parent->rb_color  = RB_BLACK;
+                gparent->rb_color = RB_RED;
+                node = gparent;
+                continue;
+            }
+            if (parent->rb_right == node) {
+                rb_rotate_left(parent, root);
+                struct rb_node* tmp = parent;
+                parent = node;
+                node = tmp;
+            }
+            parent->rb_color  = RB_BLACK;
+            gparent->rb_color = RB_RED;
+            rb_rotate_right(gparent, root);
+        } else {
+            struct rb_node* uncle = gparent->rb_left;
+            if (uncle && uncle->rb_color == RB_RED) {
+                uncle->rb_color   = RB_BLACK;
+                parent->rb_color  = RB_BLACK;
+                gparent->rb_color = RB_RED;
+                node = gparent;
+                continue;
+            }
+            if (parent->rb_left == node) {
+                rb_rotate_right(parent, root);
+                struct rb_node* tmp = parent;
+                parent = node;
+                node = tmp;
+            }
+            parent->rb_color  = RB_BLACK;
+            gparent->rb_color = RB_RED;
+            rb_rotate_left(gparent, root);
+        }
+    }
+
+    root->rb_node->rb_color = RB_BLACK;
+}
+
+// Rebalance after removing a black node. `node` may be NULL, hence the
+// explicit parent pointer.
+static void rb_erase_color(struct rb_node* node, struct rb_node* parent,
+                           struct rb_root* root) {
+    struct rb_node* other;
+
+    while ((!node || node->rb_color == RB_BLACK) && node != root->rb_node) {
+        if (parent->rb_left == node) {
+            other = parent->rb_right;
+            if (!other) break;
+
+            if (other->rb_color == RB_RED) {
+                other->rb_color  = RB_BLACK;
+                parent->rb_color = RB_RED;
+                rb_rotate_left(parent, root);
+                other = parent->rb_right;
+                if (!other) break;
+            }
+            if ((!other->rb_left  || other->rb_left->rb_color  == RB_BLACK) &&
+                (!other->rb_right || other->rb_right->rb_color == RB_BLACK)) {
+                other->rb_color = RB_RED;
+                node = parent;
+                parent = node->rb_parent;
+            } else {
+                if (!other->rb_right || other->rb_right->rb_color == RB_BLACK) {
+                    if (other->rb_left) other->rb_left->rb_color = RB_BLACK;
+                    other->rb_color = RB_RED;
+                    rb_rotate_right(other, root);
+                    other = parent->rb_right;
+                    if (!other) break;
+                }
+                other->rb_color  = parent->rb_color;
+                parent->rb_color = RB_BLACK;
+                if (other->rb_right) other->rb_right->rb_color = RB_BLACK;
+                rb_rotate_left(parent, root);
+                node = root->rb_node;
+                break;
+            }
+        } else {
+            other = parent->rb_left;
+            if (!other) break;
+
+            if (other->rb_color == RB_RED) {
+                other->rb_color  = RB_BLACK;
+                parent->rb_color = RB_RED;
+                rb_rotate_right(parent, root);
+                other = parent->rb_left;
+                if (!other) break;
+            }
+            if ((!other->rb_left  || other->rb_left->rb_color  == RB_BLACK) &&
+                (!other->rb_right || other->rb_right->rb_color == RB_BLACK)) {
+                other->rb_color = RB_RED;
+                node = parent;
+                parent = node->rb_parent;
+            } else {
+                if (!other->rb_left || other->rb_left->rb_color == RB_BLACK) {
+                    if (other->rb_right) other->rb_right->rb_color = RB_BLACK;
+                    other->rb_color = RB_RED;
+                    rb_rotate_left(other, root);
+                    other = parent->rb_left;
+                    if (!other) break;
+                }
+                other->rb_color  = parent->rb_color;
+                parent->rb_color = RB_BLACK;
+                if (other->rb_left) other->rb_left->rb_color = RB_BLACK;
+                rb_rotate_right(parent, root);
+                node = root->rb_node;
+                break;
+            }
+        }
+    }
+
+    if (node) node->rb_color = RB_BLACK;
+}
+
+void rb_erase(struct rb_node* node, struct rb_root* root) {
+    struct rb_node *child, *parent;
+    int color;
+
+    if (!node->rb_left) {
+        child = node->rb_right;
+    } else if (!node->rb_right) {
+        child = node->rb_left;
+    } else {
+        // Two children: splice in the in-order successor.
+        struct rb_node* old = node;
+        struct rb_node* left;
+
+        node = node->rb_right;
+        while ((left = node->rb_left) != NULL) node = left;
+
+        // Put the successor where `old` used to hang.
+        if (old->rb_parent) {
+            if (old->rb_parent->rb_left == old) old->rb_parent->rb_left = node;
+            else                                old->rb_parent->rb_right = node;
+        } else {
+            root->rb_node = node;
+        }
+
+        child  = node->rb_right;
+        parent = node->rb_parent;
+        color  = node->rb_color;
+
+        if (parent == old) {
+            // The successor is `old`'s direct right child, so it keeps its own
+            // right subtree and becomes the parent of the deficient side.
+            parent = node;
+        } else {
+            if (child) child->rb_parent = parent;
+            parent->rb_left = child;
+
+            node->rb_right = old->rb_right;
+            old->rb_right->rb_parent = node;
+        }
+
+        node->rb_parent = old->rb_parent;
+        node->rb_color  = old->rb_color;
+        node->rb_left   = old->rb_left;
+        old->rb_left->rb_parent = node;
+
+        if (color == RB_BLACK) rb_erase_color(child, parent, root);
+        return;
+    }
+
+    parent = node->rb_parent;
+    color  = node->rb_color;
+
+    if (child) child->rb_parent = parent;
+    if (parent) {
+        if (parent->rb_left == node) parent->rb_left = child;
+        else                         parent->rb_right = child;
+    } else {
+        root->rb_node = child;
+    }
+
+    if (color == RB_BLACK) rb_erase_color(child, parent, root);
+}
+
+struct rb_node* rb_first(const struct rb_root* root) {
+    struct rb_node* n = root->rb_node;
+    if (!n) return NULL;
+    while (n->rb_left) n = n->rb_left;
+    return n;
+}
+
+struct rb_node* rb_last(const struct rb_root* root) {
+    struct rb_node* n = root->rb_node;
+    if (!n) return NULL;
+    while (n->rb_right) n = n->rb_right;
+    return n;
+}
+
+struct rb_node* rb_next(const struct rb_node* node) {
+    if (!node) return NULL;
+
+    if (node->rb_right) {
+        struct rb_node* n = node->rb_right;
+        while (n->rb_left) n = n->rb_left;
+        return n;
+    }
+
+    struct rb_node* parent;
+    while ((parent = node->rb_parent) && node == parent->rb_right) {
+        node = parent;
+    }
+    return parent;
+}
+
+struct rb_node* rb_prev(const struct rb_node* node) {
+    if (!node) return NULL;
+
+    if (node->rb_left) {
+        struct rb_node* n = node->rb_left;
+        while (n->rb_right) n = n->rb_right;
+        return n;
+    }
+
+    struct rb_node* parent;
+    while ((parent = node->rb_parent) && node == parent->rb_left) {
+        node = parent;
+    }
+    return parent;
+}
+
+size_t rb_count(const struct rb_root* root) {
+    size_t count = 0;
+    for (struct rb_node* n = rb_first(root); n; n = rb_next(n)) count++;
+    return count;
+}
+
+// Returns the number of black nodes on any root-to-leaf path, or -1 when the
+// black-height invariant is violated.
+static int rb_black_height_of(const struct rb_node* node) {
+    if (!node) return 1;
+
+    int left  = rb_black_height_of(node->rb_left);
+    int right = rb_black_height_of(node->rb_right);
+    if (left < 0 || right < 0 || left != right) return -1;
+
+    if (node->rb_color == RB_RED) {
+        if ((node->rb_left  && node->rb_left->rb_color  == RB_RED) ||
+            (node->rb_right && node->rb_right->rb_color == RB_RED)) {
+            return -1;
+        }
+        return left;
+    }
+    return left + 1;
+}
+
+int rb_black_height(const struct rb_root* root) {
+    return rb_black_height_of(root->rb_node);
+}
+
+bool rb_validate(const struct rb_root* root) {
+    if (!root->rb_node) return true;
+    if (root->rb_node->rb_color != RB_BLACK) return false;
+    if (root->rb_node->rb_parent != NULL) return false;
+    return rb_black_height_of(root->rb_node) > 0;
+}