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.

scripts/build-wasm.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+# Build the WebAssembly target (openra3.html/.js/.wasm) with the Emscripten image.
+#
+# Browsers forbid synchronous on-demand file reads on the main thread, so assets
+# are preloaded into the module's filesystem. Prefer a compact, per-map set: one
+# map plus the tiles it uses:
+#
+#   OPENRA3_WEB_ASSETS="$(mktemp -d)" \
+#     openra3 textures --map map_mp_2_feasel1   # copy these under the set's terrain/
+#
+# then point OPENRA3_WEB_ASSETS at that directory.
+set -euo pipefail
+
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+IMAGE="${OPENRA3_WASM_IMAGE:-openra3-wasm:local}"
+BUILD_DIR="${1:-build/wasm}"
+
+docker build --target dev -f "$ROOT/Dockerfile.wasm" -t "$IMAGE" "$ROOT"
+
+mounts=(-v "$ROOT:/work")
+configure=(-DCMAKE_TOOLCHAIN_FILE=cmake/toolchains/emscripten-wasm.cmake -DCMAKE_BUILD_TYPE=Release)
+if [ -n "${OPENRA3_WEB_ASSETS:-}" ]; then
+    mounts+=(-v "$OPENRA3_WEB_ASSETS:/assets:ro")
+    configure+=(-DOPENRA3_WEB_ASSETS=/assets)
+fi
+
+docker run --rm "${mounts[@]}" -w /work "$IMAGE" cmake -S . -B "$BUILD_DIR" -G Ninja "${configure[@]}"
+docker run --rm "${mounts[@]}" -w /work "$IMAGE" cmake --build "$BUILD_DIR" -j
+
+echo "WebAssembly: $ROOT/$BUILD_DIR/bin/openra3.html (+ .js/.wasm/.data)"
+echo "serve:  python3 \"$ROOT/apps/web/serve.py\" --root \"$ROOT/$BUILD_DIR/bin\" --port 8199"