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.

shaders/dx_terrain.hlsl

@@ -130,17 +130,19 @@ float4 PSMain(VSOut input) : SV_Target {
         return float4(sky_color(dir), 1.0);
     }
 
-    // March the heightfield (bounded work: the step grows toward the horizon).
+    // March the heightfield. The step grows quickly: the map diagonal is only
+    // ~9000 world units, so marching past ~20000 adds cost without detail, and
+    // a 6% growth rate more than doubles the worst-case iteration count.
     float t = CELL * 0.5;
     float dt = CELL * 0.5;
     float prev = t;
     bool hit = false;
     float hit_t = 0.0;
-    for (int i = 0; i < 512 && t < 60000.0; ++i) {
+    for (int i = 0; i < 256 && t < 20000.0; ++i) {
         float3 w = cam_pos + dir * t;
         if (w.x < 0.0 || w.y < 0.0 || w.x >= world_w || w.y >= world_h) {
             prev = t;
-            dt *= 1.06;
+            dt *= 1.10;
             t += dt;
             continue;
         }
@@ -155,7 +157,7 @@ float4 PSMain(VSOut input) : SV_Target {
             break;
         }
         prev = t;
-        dt *= 1.06;
+        dt *= 1.10;
         t += dt;
     }
     if (!hit) {