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/shell.c

@@ -25,7 +25,10 @@ static const char* shell_builtins[] = {
     "nslookup", "hdparm", "lspci", "speaker", "mouse", "virtinfo",
     "io_uring_test", "uname", "free", "uptime", "dmesg", "ps", "top",
     "sleep", "reboot", "poweroff", "clear", "help", "neofetch", "calc",
-    "matrix", "history", "tty", "beep", "shutdown", NULL
+    "matrix", "history", "tty", "beep", "shutdown",
+    "ktest", "rcuinfo", "futexinfo", "waitinfo", "pagecache",
+    "sort", "uniq", "cut", "tr", "rev", "tac", "nl", "seq", "diff",
+    "xxd", "du", "factor", "sum", "truncate", NULL
 };
 
 static void history_add(const char* cmd) {
@@ -81,7 +84,17 @@ static void cmd_help(void) {
     printk("    " ANSI_GREEN "dmesg / ps / top" ANSI_RESET "   - Kernel boot log & process manager\n");
     printk("    " ANSI_GREEN "calc [A] [op] [B]" ANSI_RESET " - Arithmetic calculator (+, -, *, /, %%)\n");
     printk("    " ANSI_GREEN "matrix" ANSI_RESET "             - Digital rain screen animation\n");
-    printk("    " ANSI_GREEN "reboot / shutdown" ANSI_RESET "   - Power management\n");
+    printk("    " ANSI_GREEN "reboot / shutdown" ANSI_RESET "   - Power management\n\n");
+    printk(ANSI_YELLOW "  [Kernel Core Diagnostics]\n" ANSI_RESET);
+    printk("    " ANSI_GREEN "ktest [suite]" ANSI_RESET "      - Run in-kernel self-test suites (KUnit-style)\n");
+    printk("    " ANSI_GREEN "rcuinfo / futexinfo" ANSI_RESET " - Tiny RCU grace periods & futex buckets\n");
+    printk("    " ANSI_GREEN "waitinfo" ANSI_RESET "           - Wait queue sleepers and wakeup counters\n");
+    printk("    " ANSI_GREEN "pagecache [sync]" ANSI_RESET "   - Block page cache residency & hit rate\n\n");
+    printk(ANSI_YELLOW "  [Text Processing Suite]\n" ANSI_RESET);
+    printk("    " ANSI_GREEN "sort / uniq / nl" ANSI_RESET "   - Sort, de-duplicate and number lines\n");
+    printk("    " ANSI_GREEN "cut / tr / rev / tac" ANSI_RESET " - Column, character and order transforms\n");
+    printk("    " ANSI_GREEN "diff / xxd / sum" ANSI_RESET "   - Compare, hex-dump and checksum files\n");
+    printk("    " ANSI_GREEN "seq / factor / du" ANSI_RESET "  - Number sequences, factoring, disk usage\n");
     printk(ANSI_BRIGHT_BLACK "Tip: Use TAB for autocomplete, Up/Down for command history.\n" ANSI_RESET);
 }