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

kernel/main.c

@@ -14,6 +14,11 @@
 #include <kernel/metrics.h>
 #include <kernel/bpf.h>
 #include <kernel/kobject.h>
+#include <kernel/wait.h>
+#include <kernel/futex.h>
+#include <kernel/rcu.h>
+#include <kernel/ktest.h>
+#include <mm/page_cache.h>
 #include <mm/vma.h>
 #include <drivers/canvas.h>
 #include <drivers/pty.h>
@@ -94,6 +99,7 @@ static void subos_modular_core_boot(void) {
     // Storage & Block Layer
     printk(KERN_INFO "[4/14] Initializing Block Layer & Storage Controllers... ");
     block_init();
+    page_cache_init();
     ramdisk_init();
 #if defined(__x86_64__)
     ata_init();
@@ -170,6 +176,13 @@ static void subos_modular_core_boot(void) {
     namespace_init();
     printk(ANSI_BRIGHT_GREEN "OK\n" ANSI_RESET);
 
+    // Concurrency Primitives: Wait Queues, Futexes & Tiny RCU
+    printk(KERN_INFO "[9/14] Initializing Wait Queues, Futex Hash & Tiny RCU... ");
+    wait_subsystem_init();
+    futex_init();
+    rcu_init();
+    printk(ANSI_BRIGHT_GREEN "OK\n" ANSI_RESET);
+
     // Inter-Process Communication
     printk(KERN_INFO "[9/14] Initializing IPC Engine (Pipes, MsgQueues, POSIX SHM, Semaphores)... ");
     ipc_init();
@@ -209,6 +222,8 @@ static void subos_modular_core_boot(void) {
     printk(KERN_INFO "[13/14] Initializing Preemptive Task Scheduler & LazyBox... ");
     sched_init();
     lazybox_init();
+    ktest_init();
+    ktest_register_builtin_suites();
     printk(ANSI_BRIGHT_GREEN "OK\n" ANSI_RESET);
 
     // Hardware Interrupts