v0.7 line: multi-window support, fs::path / LoadResult, tc_* MCP tools

[tettou771]

Multi-window support

TrussC apps can now open multiple native windows — each with its own App,
its own input, its own ImGui, ticking at its own display's refresh rate:

// main App
void tcApp::setup() {
    WindowSettings ws;
    ws.setSize(520, 380);
    ws.setTitle("second window");
    second_ = createWindow(ws);          // shared_ptr<Window>
    second_->setApp(make_shared<SubApp>());
}

// SubApp drives the second window with the familiar App lifecycle:
// setup()/update()/draw(), keyPressed/mousePressed/..., windowResized().
class SubApp : public App {
    void setup() override {
        imgui::imguiSetup();        // its own Dear ImGui, bound to this window
        cam_.enableMouseInput();    // its own EasyCam, this window's mouse
    }
    void draw() override {
        clear(0.1f);
        // getMouseX()/getWidth()/getDeltaTime()... all answer for THIS window
    }
};

• Everything is per-window: mouse/keyboard state, hover/drag tracking,
  camera, delta time, ImGui context. getMouseX(), getDeltaTime(),
  imguiBegin() etc. just work inside each window's App — no new APIs to
  learn beyond createWindow() / Window::setApp().
• GPU resources are shared: Textures, Fbos, Meshes and Fonts can be used
  freely across windows (e.g. render once into an Fbo, show it in both).
• Windows tick independently on their own vsync; a fully occluded window
  skips rendering without stalling the others.
• AI/MCP integration: tc_list_windows lists open windows, and
  tc_get_screenshot / tc_save_screenshot take a window index — agents
  can see every window, not just the main one.
• Platforms: macOS, Windows, desktop Linux. On single-window platforms
  (web / iOS / Android / Raspberry Pi) createWindow() fails gracefully
  with a log message.
• New examples: windowing/multiWindowExample,
  windowing/imguiWindowsExample, windowing/easyCamWindowExample.

Under the hood, upstream sokol_app.h has been retired in favor of
sokol_app_tc.h, a self-contained fork implementing the same sapp_* API
(plus the window API) on all platforms — verified on real hardware:
mac, Windows 11, Linux, Chrome (WebGPU/WebGL2), iPhone, Pixel, Raspberry Pi 5.

Breaking changes

• fs::path end-to-end. Every path-carrying API now takes and
  returns fs::path. Passing strings keeps compiling (implicit
  conversion); code that assigns results to std::string needs
  .string():
  • getExecutablePath() / getExecutableDir() (the latter no longer has a
    trailing slash — join with /)
  • FileDialogResult.filePath / .fileName
  • the getPath() family (SoundPlayer, VideoPlayer, VideoWriter,
    ScreenRecorder, recordingPath()), systemFontPath()
  • This closes the long-standing UTF-8 hole on Windows: non-ASCII install
    paths now survive from the OS all the way through getDataPath().
• LoadResult. Load APIs (Image, Pixels, SoundBuffer, Sound,
  SoundStream, VideoPlayer, Font, tcxHap) return
  trussc::LoadResult — a LoadError kind plus a human-readable message —
  instead of bool. if (img.load(...)) works unchanged
  (explicit operator bool); bool ok = img.load(...) becomes
  img.load(...).ok(). Error taxonomy will grow without further breakage.
• MCP tools renamed to tc_* / tcx_imgui_* (tc_get_screenshot,
  tc_save_screenshot, tc_quit, ...). MCP clients must switch to the new
  names.
• Explicit VecN splat constructors (Vec3 v(1.0f) needs to be
  intentional) + Vec4 component-wise operators.

Also in this line

• tcxLua: generated bindings sharded across 16 TUs — peak compiler memory
  drops from ~8 GB (unbuildable on a Raspberry Pi 5) to ~1.3 GB per TU
• iOS simulator support (metal_sim shader variant) — iOS apps now run in
  the simulator, so they are AI-verifiable via simctl
• tc_get_screenshot gained format/width/quality (JPEG thumbnails,
  encoding on the HTTP worker — polling doesn't stutter the app);
  kiosk groundwork: tc_get_health, tc_get_status(_image),
  TRUSSC_LOG_FILE
• emscripten 6.x runtime fix (-sGROWABLE_ARRAYBUFFERS=0)
• Windows file dialogs use native wide strings; assorted small fixes

Verification

• Full CI green on every landing (macOS / Windows / Linux / web / Android /
  iOS compile, 108-example sweep, core tests, HotReload, reference check)
• Real-device passes recorded in docs/dev/sokol_app_tc-design.md
  (per-platform behavioral contracts in docs/dev/sapp-*-impl-spec.md)

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