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

Makefile

@@ -126,7 +126,8 @@ CORE_SRCS = kernel/main.c kernel/task.c kernel/sync.c kernel/timer.c kernel/prin
             kernel/syslog.c kernel/module.c kernel/kobject.c kernel/metrics.c kernel/namespace.c \
             kernel/signal.c kernel/workqueue.c kernel/tsc.c kernel/vt_art.c \
             kernel/sub/sub_runtime.c kernel/sub/sub_vm.c \
-            lib/string.c lib/vsprintf.c lib/bitmap.c \
+            kernel/wait.c kernel/futex.c kernel/rcu.c kernel/ktest.c \
+            lib/string.c lib/vsprintf.c lib/bitmap.c lib/rbtree.c lib/kfifo.c lib/hashtable.c \
             init/cmdline.c init/service.c usr/initramfs.c block/block.c block/elevator.c \
             ipc/sem.c ipc/shm.c ipc/shm_posix.c ipc/pipe.c ipc/ipc.c ipc/msg.c \
             crypto/sha256.c crypto/md5.c crypto/crc32.c crypto/prng.c sound/beep.c
@@ -170,7 +171,7 @@ endif
 CONFIG_SRCS-y := $(CORE_SRCS)
 
 ifeq ($(CONFIG_MM_PMM), y)
-    CONFIG_SRCS-y += mm/pmm.c mm/kmalloc.c
+    CONFIG_SRCS-y += mm/pmm.c mm/kmalloc.c mm/page_cache.c
 endif
 ifeq ($(CONFIG_MM_SLAB), y)
     CONFIG_SRCS-y += mm/slab.c
@@ -279,7 +280,7 @@ CONFIG_SRCS-y += drivers/char/tty.c drivers/char/keyboard.c drivers/char/serial.
 
 # Userland
 ifeq ($(CONFIG_USERLAND_LAZYBOX), y)
-    CONFIG_SRCS-y += userland/lazybox/lazybox.c userland/lazybox/shell.c userland/lazybox/sh.c userland/lazybox/nano.c userland/lazybox/snake.c userland/lazybox/tree.c
+    CONFIG_SRCS-y += userland/lazybox/lazybox.c userland/lazybox/shell.c userland/lazybox/sh.c userland/lazybox/nano.c userland/lazybox/snake.c userland/lazybox/tree.c userland/lazybox/coreutils.c
 endif
 
 # Deduplicate