v0.6.0: WebAssembly build with a WebGL backend (playable in the browser)

- ra3.webgl: a Web peer of Vulkan/Direct3D (SDL3 canvas + GLES3/WebGL2), with
  2D blit and the GPU terrain raymarch; GLSL ES shaders; null fallback off
  Emscripten. backend::webgl + default_backend() (webgl under Emscripten).
- Emscripten toolchain + Dockerfile.wasm + scripts/build-wasm.sh; import std,
  global -fexceptions, and Asyncify so the blocking frame loop yields to the
  browser (display::sleep_frame calls emscripten_sleep).
- apps/web/shell.html + apps/web/serve.py (Range-capable dev server).
- Assets: browsers forbid synchronous on-demand reads on the main thread (and
  FS.createLazyFile / the WasmFS fetch backend are worker-only; SDL3's Emscripten
  backend is main-thread DOM only, so the engine cannot run in a worker). The
  wasm build therefore preloads a compact per-map set via OPENRA3_WEB_ASSETS.
- New openra3 textures --map ID lists the loose terrain TGAs a map resolves to
  (terrain::resolve_texture_files, shared with load_textures_from_dir).
- Fixes: webgl heightmap used GL_R16 = 0x8229 (that is R8) -> upload rejected,
  terrain flattened to water; now 0x822A. Logger uses a stdout console sink on
  the web (stderr maps to console.error); a GL error check logs bad uploads.
- CI: wasm image + build jobs.

src/client/ra3.client.cppm

@@ -1,3 +1,10 @@
+module;
+
+#if defined(__EMSCRIPTEN__)
+// Yields to the browser (requires -sASYNCIFY); see display::sleep_frame.
+extern "C" void emscripten_sleep(unsigned int ms);
+#endif
+
 export module ra3.client;
 
 import std;
@@ -85,12 +92,19 @@ export namespace ra3::client {
 
         /** Throttle an idle frame according to the configured limit. */
         auto sleep_frame() const -> void {
+#if defined(__EMSCRIPTEN__)
+            // The frame loops are blocking; on the web the only way to let the
+            // browser paint and handle input is to yield (Asyncify sleep).
+            const int fps = fps_limit_ > 0 ? fps_limit_ : 60;
+            emscripten_sleep(static_cast<unsigned>(std::max(1, 1000 / fps)));
+#else
             if (fps_limit_ > 0) {
                 std::this_thread::sleep_for(std::chrono::milliseconds(std::max(1, 1000 / fps_limit_)));
             } else if (fps_limit_ < 0) {
                 std::this_thread::yield();
             }
             // fps_limit_ == 0: vertical sync already blocks in present().
+#endif
         }
 
         // ---- shared interactive loops (assume `init` succeeded) --------------