Expand voxel gameplay, lighting, full-height streaming and world imports

Add shared Rust/WASM physics, worker meshing and diagnostics, 64-chunk full-height streaming, atlas texture support, and baseline world import. Document the current implementation and include the supplied in-game lobby screenshot.

docs/PHYSICS_PLAN.md

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+# Java 26.2 physics implementation
+
+The target is measured Java 26.2 player movement, using one original fixed-tick
+Rust implementation on the server and in the browser through WebAssembly.
+Existing texture, chunk-streaming, and remote-rotation improvements must remain.
+
+1. Measure movement attributes, block friction and speed/jump factors, poses,
+   and available reference trajectories from the pinned official executable.
+2. Build the shared movement solver: velocity, acceleration, friction, gravity,
+   jumping, sprinting, sneak-edge protection, swept collisions, step-up, poses,
+   climbing, fluids, special surfaces, and permitted creative flight.
+3. Integrate server-authoritative commands, processed acknowledgments, and
+   bounded client prediction/replay. Preserve package jump hooks and arena rules.
+4. Verify original reference facts, trajectory behavior, native/WASM parity,
+   packet ordering, latency recovery, and the running browser with the local pack.
+5. Record measured compatibility and explicit remaining mechanics. Movement
+   parity must not be confused with complete vanilla AI, redstone, or world-fluid
+   simulation. Additional interactions must be described by their actual tests.
+
+Progress and final evidence belong in `docs/PHYSICS.md` and ignored
+`artifacts/physics/`. Do not describe simulated reference formulas as measurements
+from the official executable.