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

userland/lazybox/lazybox.c

@@ -38,6 +38,15 @@
 #include <kernel/tsc.h>
 #include <kernel/vt_art.h>
 #include <drivers/cpufreq.h>
+#include <userland/coreutils.h>
+#include <kernel/ktest.h>
+#include <kernel/rcu.h>
+#include <kernel/futex.h>
+#include <kernel/wait.h>
+#include <mm/page_cache.h>
+#include <lib/hashtable.h>
+#include <lib/kfifo.h>
+#include <lib/rbtree.h>
 #include <drivers/virtio_gpu.h>
 #include <drivers/e1000e.h>
 #include <drivers/virtio_input.h>
@@ -2023,6 +2032,62 @@ static int applet_sensors(int argc, char** argv) {
     return 0;
 }
 
+// -------------------------------------------------------------
+// Kernel Core Diagnostics: KTest, RCU, Futex, Wait Queues, Page Cache
+// -------------------------------------------------------------
+static int applet_ktest(int argc, char** argv) {
+    if (argc >= 2 && strcmp(argv[1], "-l") == 0) {
+        ktest_list_suites();
+        return 0;
+    }
+
+    ktest_result_t result;
+    int failures = (argc >= 2) ? ktest_run_suite(argv[1], &result)
+                               : ktest_run_all(&result);
+    return (failures > 0) ? 1 : 0;
+}
+
+static int applet_rcuinfo(int argc, char** argv) {
+    if (argc >= 2 && strcmp(argv[1], "-s") == 0) {
+        printk("Forcing an RCU grace period...\n");
+        rcu_synchronize();
+        printk(ANSI_BRIGHT_GREEN "Grace period #%llu completed\n" ANSI_RESET,
+               (unsigned long long)rcu_get_grace_period());
+        return 0;
+    }
+    rcu_dump();
+    return 0;
+}
+
+static int applet_futexinfo(int argc, char** argv) {
+    futex_dump();
+    return 0;
+}
+
+static int applet_waitinfo(int argc, char** argv) {
+    wait_dump_stats();
+    return 0;
+}
+
+static int applet_pagecache(int argc, char** argv) {
+    if (argc >= 2) {
+        if (strcmp(argv[1], "sync") == 0) {
+            int n = page_cache_sync();
+            printk("page cache: %d dirty page(s) written back\n", n);
+            return 0;
+        }
+        if (strcmp(argv[1], "drop") == 0) {
+            page_cache_drop_all();
+            printk("page cache: flushed and dropped all clean pages\n");
+            return 0;
+        }
+        printk("Usage: pagecache [sync|drop]\n");
+        return 1;
+    }
+    page_cache_dump();
+    return 0;
+}
+
 // -------------------------------------------------------------
 // Applet Dispatch Table (Over 70 Linux Tools)
 // -------------------------------------------------------------
@@ -2178,6 +2243,29 @@ static const lazybox_applet_t applets[] = {
     {"gpuinfo",       applet_gpuinfo,       "gpuinfo",                   "VirtIO-GPU hardware status", "Virtualization"},
     {"e1000e",        applet_e1000e,        "e1000e",                    "Intel e1000e PCIe NIC stats", "Network"},
 
+    // Kernel Core Diagnostics
+    {"ktest",         applet_ktest,         "ktest [suite|-l]",          "Run in-kernel self-test suites", "Kernel"},
+    {"rcuinfo",       applet_rcuinfo,       "rcuinfo [-s]",              "Tiny RCU grace period status", "Kernel"},
+    {"futexinfo",     applet_futexinfo,     "futexinfo",                 "Futex hash bucket telemetry", "Kernel"},
+    {"waitinfo",      applet_waitinfo,      "waitinfo",                  "Wait queue & sleeper statistics", "Kernel"},
+    {"pagecache",     applet_pagecache,     "pagecache [sync|drop]",     "Block page cache & hit rate", "Storage"},
+
+    // Text Processing Suite (GNU coreutils compatible)
+    {"sort",          coreutils_sort,       "sort [-rnfu] <file>",       "Sort lines of a text file",  "Filesystem"},
+    {"uniq",          coreutils_uniq,       "uniq [-cdu] <file>",        "Filter adjacent duplicate lines", "Filesystem"},
+    {"cut",           coreutils_cut,        "cut -f <n> [-d c] <file>",  "Extract columns from lines", "Filesystem"},
+    {"tr",            coreutils_tr,         "tr <set1> <set2> <file>",   "Translate or delete characters", "Filesystem"},
+    {"rev",           coreutils_rev,        "rev <file>",                "Reverse characters of each line", "Filesystem"},
+    {"tac",           coreutils_tac,        "tac <file>",                "Print file lines in reverse", "Filesystem"},
+    {"nl",            coreutils_nl,         "nl [-a] <file>",            "Number the lines of a file", "Filesystem"},
+    {"seq",           coreutils_seq,        "seq [first [step]] last",   "Print a sequence of numbers", "Core"},
+    {"diff",          coreutils_diff,       "diff <file1> <file2>",      "Compare files line by line", "Filesystem"},
+    {"xxd",           coreutils_xxd,        "xxd [-l n] <file>",         "Canonical hex + ASCII dump", "Filesystem"},
+    {"du",            coreutils_du,         "du [-s] [path]",            "Estimate file space usage",  "Storage"},
+    {"factor",        coreutils_factor,     "factor <number...>",        "Print prime factorization",  "Core"},
+    {"sum",           coreutils_sum,        "sum <file>",                "BSD 16-bit rotating checksum", "Crypto"},
+    {"truncate",      coreutils_truncate,   "truncate -s <n> <file>",    "Shrink or extend a file",    "Filesystem"},
+
     {NULL, NULL, NULL, NULL, NULL}
 };