feat(rust,gui): Rust checksum engine + Game of Life desktop app

Rust kernel expansion:
- rust/src/checksum/mod.rs (new): memory-safe CRC-32C (Castagnoli), Adler-32,
  and FNV-1a-64 with thin C-ABI shims. Registered in lib.rs with a boot-time
  self-test (crc32c("SUB-OS")=0x61e96ce9, verified against a reference impl).
- include/kernel/rust.h: FFI declarations; rust/rust_fallback.c: matching C
  fallbacks so aarch64/armv8i (no native rustc target) get identical results.
- LazyBox: new `cksum <text>` applet printing all three via the Rust engine.

GUI expansion:
- gui/gui_apps.c: Conway's Game of Life -- an interactive B3/S23 cellular
  automaton with Play/Pause/Step/Random/Clear toolbar, click-to-toggle cells,
  a live generation/population readout, and a board that scales with the
  window. The Random button seeds from the Rust ChaCha20 CSPRNG. Toolbar and
  cell input are gated on the MOUSE_DOWN edge so they do not re-fire while held.
- Registered "Life" (flask icon) in the desktop app table and on the desktop.

Verified: clean build on x86_64/aarch64/armv8i; boots through the SUB login to
the desktop (screenshotted) with the checksum self-test and 0 panics/0 OOM.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Fh5z6V7nGPbHvqVG5p4CXX

userland/lazybox/lazybox.c

@@ -463,6 +463,25 @@ static int applet_crc32(int argc, char** argv) {
     return 0;
 }
 
+static int applet_cksum(int argc, char** argv) {
+    const char* str = (argc >= 2) ? argv[1] : "";
+    size_t len = strlen(str);
+    const uint8_t* buf = (const uint8_t*)str;
+
+    // All three come from the memory-safe Rust checksum engine
+    // (rust/src/checksum/mod.rs), with a C bridge on non-x86 targets.
+    uint32_t crc = rust_crc32c(buf, len);
+    uint32_t adl = rust_adler32(buf, len);
+    uint64_t fnv = rust_fnv1a64(buf, len);
+
+    printk(ANSI_BRIGHT_CYAN "Rust checksum engine" ANSI_RESET " for \"%s\" (%llu bytes):\n",
+           str, (unsigned long long)len);
+    printk("  CRC-32C   : 0x%08X\n", crc);
+    printk("  Adler-32  : 0x%08X\n", adl);
+    printk("  FNV-1a-64 : 0x%016llX\n", (unsigned long long)fnv);
+    return 0;
+}
+
 static int applet_rand(int argc, char** argv) {
     int count = (argc >= 2) ? atoi(argv[1]) : 4;
     printk("Random Integers: ");
@@ -2170,6 +2189,7 @@ static const lazybox_applet_t applets[] = {
     {"md5sum",        applet_md5sum,        "md5sum <file|text>",        "Compute MD5 hash",           "Crypto"},
     {"sha256sum",     applet_sha256sum,     "sha256sum <file|text>",     "Compute SHA-256 hash",       "Crypto"},
     {"crc32",         applet_crc32,         "crc32 <text>",              "Calculate CRC32 checksum",   "Crypto"},
+    {"cksum",         applet_cksum,         "cksum <text>",              "Rust CRC-32C/Adler-32/FNV",  "Crypto"},
     {"rand",          applet_rand,          "rand [count]",              "Generate random numbers",    "Crypto"},
     {"certcheck",     applet_certcheck,     "certcheck",                 "View X.509 kernel keyring",  "Security"},
     {"capsh",         applet_capsh,         "capsh",                     "View process capabilities",  "Security"},