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

gui/gui_desktop.c

@@ -56,6 +56,7 @@ static const desktop_app_t g_apps[] = {
     { "Editor",     GUI_ICON_FILE,     gui_app_editor_launch,   120,  50, 500, 330 },
     { "Calculator", GUI_ICON_CALC,     gui_app_calc_launch,     260, 100, 220, 240 },
     { "Paint",      GUI_ICON_PAINT,    gui_app_paint_launch,    200,  60, 360, 260 },
+    { "Life",       GUI_ICON_FLASK,    gui_app_life_launch,     240,  70, 480, 380 },
     { "Clock",      GUI_ICON_CLOCK,    gui_app_clock_launch,    460, 120, 280, 260 },
     { "Settings",   GUI_ICON_SETTINGS, gui_app_settings_launch, 160,  60, 400, 320 },
     { "About",      GUI_ICON_INFO,     gui_app_about_launch,    240, 140, 330, 170 },
@@ -63,7 +64,8 @@ static const desktop_app_t g_apps[] = {
 #define APP_COUNT ((int)(sizeof(g_apps) / sizeof(g_apps[0])))
 
 // Which of the above appear on the desktop itself.
-static const int g_desktop_icons[] = { 0, 1, 2, 4, 5, 10 };
+// Indices into g_apps[]: Terminal, Files, Monitor, Kernel Log, KTest, Life, Settings.
+static const int g_desktop_icons[] = { 0, 1, 2, 4, 5, 9, 11 };
 #define DESKTOP_ICON_COUNT ((int)(sizeof(g_desktop_icons) / sizeof(g_desktop_icons[0])))
 
 static bool g_start_menu_open   = false;