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.

.gitlab-ci.yml

@@ -291,4 +291,44 @@ build_debian_arm64:
     name: "$CI_PROJECT_NAME-debian-arm64-$CI_COMMIT_SHORT_SHA"
     paths:
       - artifacts/debian-arm64/
+    expire_in: 7 days
+# --- WebAssembly (Emscripten) -------------------------------------------------
+wasm_image:
+  stage: image
+  image: docker:latest
+  services: *dind
+  tags:
+    - docker
+  before_script: *retry
+  script:
+    - retry sh -c 'echo "$HARBOR_PASSWORD" | docker login "$HARBOR_HOST" -u "$HARBOR_USERNAME" --password-stdin'
+    - |
+      retry docker build \
+        --cache-from type=registry,ref=$IMAGE:cache-wasm,insecure=true \
+        --cache-to type=registry,ref=$IMAGE:cache-wasm,mode=max,insecure=true \
+        --build-arg APT_MIRROR=$APT_MIRROR \
+        --target dev \
+        -t $IMAGE:$CI_COMMIT_SHORT_SHA-wasm \
+        -f Dockerfile.wasm .
+    - retry docker push $IMAGE:$CI_COMMIT_SHORT_SHA-wasm
+
+build_wasm:
+  stage: build
+  image: docker:latest
+  services: *dind
+  tags:
+    - docker
+  needs:
+    - wasm_image
+  before_script: *retry
+  script:
+    - retry sh -c 'echo "$HARBOR_PASSWORD" | docker login "$HARBOR_HOST" -u "$HARBOR_USERNAME" --password-stdin'
+    - retry docker pull $IMAGE:$CI_COMMIT_SHORT_SHA-wasm
+    - docker run --rm -v "$(pwd):/work" -w /work $IMAGE:$CI_COMMIT_SHORT_SHA-wasm cmake -S . -B build/wasm -G Ninja -DCMAKE_TOOLCHAIN_FILE=cmake/toolchains/emscripten-wasm.cmake -DCMAKE_BUILD_TYPE=Release
+    - docker run --rm -v "$(pwd):/work" -w /work $IMAGE:$CI_COMMIT_SHORT_SHA-wasm cmake --build build/wasm -j
+    - docker run --rm -v "$(pwd):/work" -w /work $IMAGE:$CI_COMMIT_SHORT_SHA-wasm bash -c "mkdir -p artifacts/wasm && cp build/wasm/bin/openra3.html build/wasm/bin/openra3.js build/wasm/bin/openra3.wasm build/wasm/bin/openra3.data artifacts/wasm/ 2>/dev/null || cp build/wasm/bin/openra3.html build/wasm/bin/openra3.js build/wasm/bin/openra3.wasm artifacts/wasm/"
+  artifacts:
+    name: "$CI_PROJECT_NAME-wasm-$CI_COMMIT_SHORT_SHA"
+    paths:
+      - artifacts/wasm/
     expire_in: 7 days