Implement tiled multiplayer city simulation and scaling stages

client/README.md

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+# Browser client
+
+`tiles.ts` loads prepared city tiles over HTTP. The existing renderer and offline
+debug generator remain in `src/` to preserve the working prototype.
+
+Open `/?city=/cities/demo/manifest.json`. Camera movement loads only intersecting
+tiles plus a margin, with six concurrent requests and a bounded tile cache.