docs: crater erosion modes and the retired 1.2x exclusion (terrain-water task 19)

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Tools/Scripts/README.md

@@ -53,7 +53,18 @@ Generates the entire 2D blueprint. Roughly in order:
    `ErosionDepositCap`; both caps are per-cell metres against one net-displacement ledger and are
    asserted on exit), a sea clamp forbids carving below sea + margin and leaves below-sea cells
    untouched in both directions (the rendered coastline cannot move — asserted every generation),
-   and nothing within 1.2× the crater radius is modified. Output-height only: biome/water
+   and the crater is handled by `CraterErosionMode` (task 19): nothing inside the protected strike
+   core (`CraterErosionCore`, **0.80 ×** CraterRadius — the carve's own extent) is modified, and
+   outside it either `"full"` applies full strength at once or `"feather"` ramps 0→full out to
+   `CraterErosionFeather` (1.05 ×). Task 17's hard 1.2 × cutoff was replaced because it held
+   **620 811 land cells** of ordinary terrain smooth for no geometric reason — measured, the carve's
+   displacement is exactly 0 beyond 0.80 × — which read as an un-eroded disc with a hard edge.
+   Keeping the core at the carve radius is what makes erosion and the carve touch **disjoint**
+   cells; a narrower core lets the carve, which runs afterwards and scales height toward the sea
+   target, amplify erosion's deltas across the waterline (measured at a 0.50 core: 79 rendered
+   waterline crossings). The flooded bay and its sea connection never depend on the crater mode —
+   below-sea cells are read-only in both directions, so the bay can be neither carved open nor
+   silted shut. Output-height only: biome/water
    classification reads the retained pre-erosion map, so `1_biomes`/`0_water` are bit-identical
    with erosion on or off.
    The task-18 defaults tune for a drainage **hierarchy** — fine rills everywhere feeding a set of