Trade sorted transparency for stable coverage.

Alpha hashing turns a fractional opacity into a screen-door coverage decision. This instanced field keeps the material policy visible beside temporal accumulation, so `alphaHash`, `transparent`, `depthWrite`, and TAA are distinct switches.

This route preserves the source-backed alpha-hash/TAA contract with deterministic local instances and a RoomEnvironment. The official instance count, exact page/pixel fidelity, Stats/gui presentation, device/compiler coverage, and whole-example fidelity remain unverified.