Lake banks slope down to the water instead of ending in a cliff (t96)

Digging basins to the lower rim left a 9-18 grain vertical cliff on each lake's high side
(owner screenshots, seed 766733). From each edge inward the ground now falls at most 0.7 grains
a column and is never raised above the natural surface. New no-cliff check in t_lakes.js;
world hashes re-baselined (shallow ore: 2 cells changed, on a lake bank).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/DESIGN.md

@@ -607,6 +607,10 @@ above, including:
   and each lake's half-width is capped so its edge stays 80 grains from spawn (and the dock).
   Lakes are smaller and lower: about 180-250 grains wide instead of 250-330, 5-15 grains
   lower. The random draws keep their order.
+  Dug to the lower rim, the high side ended in a vertical cliff as tall as the gap between the
+  rims (15-18 grains on seeds 766733, 7, 42; owner screenshots). From each edge inward the
+  ground may now fall at most 0.7 grains a column and is never raised above the natural
+  surface, so the high side is a sloped bank down to the water.
 
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