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

include/userland/coreutils.h

@@ -0,0 +1,22 @@
+#ifndef _USERLAND_COREUTILS_H
+#define _USERLAND_COREUTILS_H
+
+// GNU-style text-processing applets for LazyBox. Each entry point matches
+// applet_fn_t so it can be placed directly in the LazyBox dispatch table.
+
+int coreutils_sort(int argc, char** argv);
+int coreutils_uniq(int argc, char** argv);
+int coreutils_cut(int argc, char** argv);
+int coreutils_tr(int argc, char** argv);
+int coreutils_rev(int argc, char** argv);
+int coreutils_tac(int argc, char** argv);
+int coreutils_nl(int argc, char** argv);
+int coreutils_seq(int argc, char** argv);
+int coreutils_diff(int argc, char** argv);
+int coreutils_xxd(int argc, char** argv);
+int coreutils_du(int argc, char** argv);
+int coreutils_factor(int argc, char** argv);
+int coreutils_sum(int argc, char** argv);
+int coreutils_truncate(int argc, char** argv);
+
+#endif // _USERLAND_COREUTILS_H