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.

Dockerfile.wasm

@@ -0,0 +1,46 @@
+# syntax=docker/dockerfile:1
+
+# WebAssembly build environment (Emscripten), producing openra3.html/.js/.wasm.
+#
+# The emsdk image ships CMake 3.28, whose experimental `import std` support is
+# too old for the project, so an upstream CMake is layered on top.
+
+FROM emscripten/emsdk:6.0.10 AS dev
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+ARG APT_MIRROR=""
+RUN if [ -n "$APT_MIRROR" ]; then \
+        sed -i -e "s|http://archive.ubuntu.com/ubuntu|${APT_MIRROR}|g" \
+               -e "s|http://security.ubuntu.com/ubuntu|${APT_MIRROR}|g" \
+               -e "s|http://ports.ubuntu.com/ubuntu-ports|${APT_MIRROR}|g" \
+               /etc/apt/sources.list /etc/apt/sources.list.d/*.sources 2>/dev/null || true; \
+    fi
+
+RUN --mount=type=cache,target=/var/lib/apt,sharing=locked \
+    --mount=type=cache,target=/var/cache/apt,sharing=locked \
+    for attempt in 1 2 3 4 5; do \
+        apt-get -o Acquire::Retries=5 update && break; \
+        echo "apt-get update failed (attempt $attempt), retrying"; \
+        sleep 5; \
+    done \
+    && apt-get -o Acquire::Retries=5 install -y --no-install-recommends \
+        ca-certificates \
+        curl \
+        ninja-build \
+    && rm -rf /var/lib/apt/lists/*
+
+# Upstream CMake, because the emsdk image's 3.28 cannot build the project.
+ARG CMAKE_VERSION=4.2.3
+RUN mkdir -p /opt/cmake \
+    && curl -fL "https://github.com/Kitware/CMake/releases/download/v${CMAKE_VERSION}/cmake-${CMAKE_VERSION}-linux-x86_64.tar.gz" \
+        | tar -xz -C /opt/cmake --strip-components=1
+ENV PATH="/opt/cmake/bin:${PATH}"
+
+WORKDIR /work
+COPY . .
+
+RUN cmake -S . -B build/wasm -G Ninja \
+        -DCMAKE_TOOLCHAIN_FILE=cmake/toolchains/emscripten-wasm.cmake \
+        -DCMAKE_BUILD_TYPE=Release \
+    && cmake --build build/wasm -j