v1.6.0: the water release

Deterministic particle fluids join the contract: an opt-in per-world pool (fixed capacity, physics pinned at creation) with slot-stable storage, a canonical neighbor grid, a LiquidFun-lineage relaxation solver (pressure, damping, optional viscosity), two-way rigid coupling (light things float, dense things sink), anti-tunneling projection, one-way platform and sensor laws, and wake-on-touch. Everything lives inside the same snapshot, journal and hash discipline as the rigid world: 16 gated determinism lines across 9 CI cells and 3 instruction sets, with the API fuzzer driving water through all four promises on every seed. The testbed gains a faucet-and-basin scene whose pour rewinds bit for bit; the guide gains the Water chapter. Wire v25, snapshot v28. The reference lineage is credited in THIRD_PARTY.md.