v0.7.0: SDL-free wasm worker backend (WebGPU + WebGL2) with on-demand assets

The engine now runs on a plain Web Worker (no SDL, no PROXY_TO_PTHREAD: in a pthread Emscripten proxies every filesystem syscall to the main browser thread, where synchronous XHR - and thus FS.createLazyFile - is forbidden). A page/worker pair transfers an OffscreenCanvas and forwards DOM input; assets load lazily from an embedded manifest, so entering a map fetches only that map and its tiles.

Presentation: ra3.webgpu (WebGPU via Emscripten's emdawnwebgpu port, WGSL shaders, the default on wasm) and ra3.wasmgl (WebGL2, GLSL ES). The legacy SDL ra3.webgl backend is removed.

src/webgpu/ra3.webgpu.null.cppm

@@ -0,0 +1,26 @@
+export module ra3.webgpu;
+
+import std;
+
+export import ra3.core;
+export import ra3.render;
+export import ra3.client;
+
+/**
+ * Fallback WebGPU backend for builds without Emscripten. `init` fails so the
+ * caller can select another display; the class name matches the real
+ * `ra3.webgpu` module so the backend factory in `ra3.display` compiles
+ * unchanged.
+ */
+export namespace ra3::webgpu {
+    class webgpu_display final : public ra3::client::display {
+    public:
+        [[nodiscard]] auto init(const ra3::client::display_options &) -> bool override { return false; }
+        [[nodiscard]] auto present(const ra3::render::image &, const ra3::render::view_rect &, bool) -> bool override { return false; }
+        [[nodiscard]] auto poll_event(ra3::render::ui_event &) -> bool override { return false; }
+        [[nodiscard]] auto window_size() const -> std::pair<int, int> override { return {0, 0}; }
+        [[nodiscard]] auto key_down(ra3::render::ui_key) const -> bool override { return false; }
+        auto shutdown() -> void override {}
+        [[nodiscard]] auto name() const -> std::string_view override { return "webgpu(null)"; }
+    };
+}