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.

third_party/libenderlog/src/ender.log.cppm

@@ -22,7 +22,7 @@ module;
 #define WIN32_LEAN_AND_MEAN
 #define NOMINMAX
 #include <windows.h>
-#elif defined(__unix__) || defined(__APPLE__)
+#elif (defined(__unix__) || defined(__APPLE__)) && !defined(__EMSCRIPTEN__)
 #include <execinfo.h>
 #endif
 
@@ -367,7 +367,7 @@ export namespace ender::log {
             text += std::format("    #{:<3}{}\n", index, symbolicate_frame(frames[index]));
         }
         return text;
-#elif defined(__unix__) || defined(__APPLE__)
+#elif (defined(__unix__) || defined(__APPLE__)) && !defined(__EMSCRIPTEN__)
         void *frames[max_frames] = {};
         const int count = ::backtrace(frames, static_cast<int>(std::min(max_frames, skip + std::max<std::size_t>(depth, 1U))));
         char **symbols = ::backtrace_symbols(frames, count);