feat(cpp,gui): C++ Analytics Engine + live GUI Analytics app

Kernel (C++17 freestanding):
  - New Analytics Engine (kernel/cpp/cpp_analytics.cpp): templated rolling
    time-series (TimeSeries<N>) sampling four live channels -- CPU load %,
    memory %, kernel-heap %, network pkt/interval -- with windowed statistics
    (min/max/mean/stddev) computed in pure integer math (kernel is -mno-sse, so
    no FP; stddev uses a Newton integer isqrt). Demonstrates virtual dispatch
    via an Observer pattern (IAnalyticsObserver + a ThresholdAlarmObserver that
    counts breaches) and RAII singletons.
  - Extended cpp_containers.hpp with Optional<T>, Pair<A,B>, and the kmin/kmax/
    kclamp/kswap algorithms.
  - Full C fallback (cpp_fallback.c) so the engine is functionally identical on
    aarch64/armv8i, which don't build the .cpp path.
  - Wired cpp_analytics_init() into boot [8/14]; seeds a few samples so graphs
    are populated immediately.

GUI:
  - New "Analytics" desktop app (gui/gui_apps.c): a 2x2 grid of live line-charts
    backed by the C++ engine, each with gridlines, a soft baseline fill, and
    min/avg/max(/stddev) overlays; click a panel to focus one channel full-size,
    click again to return. Throttled to ~2 Hz sampling off the compositor's
    repaint cadence. Registered on the desktop and in the start menu.

Userland: new `cppstat [samples]` applet dumps the engine's per-channel stats
and Observer breach counters.

Verified in QEMU (x86_64): boots clean on x86_64/aarch64/armv8i; `cppstat`
prints live stats (CPU 6%, Mem 7%, Heap 22%, computed min/avg/max/stddev,
Observer breaches); the GUI Analytics window renders all four graphs with the
Kernel Heap curve visibly climbing 22->55% as windows open, compositor steady
at ~53 FPS.

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

gui/gui_desktop.c

@@ -63,14 +63,15 @@ static const desktop_app_t g_apps[] = {
     { "RegEdit",    GUI_ICON_SETTINGS, gui_app_regedit_launch,  300,  90, 480, 360 },
     { "Clock",      GUI_ICON_CLOCK,    gui_app_clock_launch,    460, 120, 280, 260 },
     { "Settings",   GUI_ICON_SETTINGS, gui_app_settings_launch, 160,  60, 400, 320 },
+    { "Analytics",  GUI_ICON_MONITOR,  gui_app_analytics_launch,320,  70, 560, 400 },
     { "About",      GUI_ICON_INFO,     gui_app_about_launch,    240, 140, 330, 170 },
 };
 #define APP_COUNT ((int)(sizeof(g_apps) / sizeof(g_apps[0])))
 
 // Which of the above appear on the desktop itself.
 // Indices into g_apps[]: Terminal, Files, Monitor, Kernel Log, KTest, Life,
-// Rust Lab, Settings. (Objects/RegEdit live in the start menu.)
-static const int g_desktop_icons[] = { 0, 1, 2, 4, 5, 9, 10, 14 };
+// Rust Lab, Settings, Analytics. (Objects/RegEdit live in the start menu.)
+static const int g_desktop_icons[] = { 0, 1, 2, 4, 5, 9, 10, 14, 15 };
 #define DESKTOP_ICON_COUNT ((int)(sizeof(g_desktop_icons) / sizeof(g_desktop_icons[0])))
 
 static bool g_start_menu_open   = false;