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.

shaders/webgl_scene_frag.glsl

@@ -0,0 +1,15 @@
+#version 300 es
+// Samples the software-rendered scene image (the WebGL port of scene.frag).
+precision highp float;
+precision highp sampler2D;
+
+uniform sampler2D u_scene;
+
+in vec2 v_uv;
+out vec4 frag_color;
+
+void main() {
+    if (v_uv.x < 0.0 || v_uv.x > 1.0 || v_uv.y < 0.0 || v_uv.y > 1.0) discard;
+    // The engine stores 0xAARRGGBB (BGRA in memory), uploaded as GL_RGBA.
+    frag_color = texture(u_scene, v_uv).bgra;
+}