nx_procgen_water.nx

nx_procgen_water.nx -- CAUSAL water: source-to-sea rivers + drainage rills (PROCGEN arc P4 = EXCEED10 ladder R3, bucket B3; also the first honest attack on the grader's chronic LOCAL_VARIATION loss -- the refine verdict has said ADD_DETAIL on nearly every config since P1). Water here is DERIVED, never painted: springs -- constraint-picked sites: poisson candidates filtered to the upper-third elevation (water starts high) trace -- steepest-descent walk: each step moves to the strictly lowest 4-neighbour; termination is CAUSAL (map edge = reached the sea, local minimum = water collects) carve -- channel incision along the traced path (centre + banks), terminus local-min gets a pond basin (nx_sig_basin -- P1's primitive, composed not duplicated) rills -- a DENSE network of short shallow traces from poisson sites: drainage texture. This is what raises the LOCAL_VARIATION axis -- channel edges create real neighbour relief, the kind fbm-at-our-cell-step lacks. The water MASK (1 = channel/pond cell) feeds P3: a feature standing in water IS water (reeds/pool), overriding the biome mix -- causality beats the probability table. API is ADDITIVE (contract stability): nx_procgen_landscape (v1) is untouched; nx_procgen_landscape_v2 composes v1 + water and fills the caller's mask. All integer Q10; deterministic per (seed, preset). license_tier: ORIGINAL