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>

test/t_corner_seal.js

@@ -6,9 +6,9 @@ const { check, done } = suite();
 
 // t96 re-baseline: only lake generation (rim height, spawn clearance, levelling) changes these material bytes.
 for (const [seed, expected] of [
-  [777, '7d60a0f1a92b42438471c8c15c5cc92576a75a4aa54b8c519c42904acb7088a6'],
-  [77, '724eb1730ca5fead6aeaa13994beb2e066bb9d65c7db7e0b790857e2d72b7c2a'],
-  [4242, '9ffdabf1ffe8ab6cd5855ccfe05c31221d1a0517d960bf2e2273018e9dbf03f9'],
+  [777, 'dc0a19b076f341bcd944e1692651446198f57c7353db5fdea36d6bdd886ab6ff'],
+  [77, 'aa979290d81a758e6ff68fa22b06513fb546aa73a80d09e69b35899a46758a0b'],
+  [4242, '25737574612fe0395a2b0304a1ac698f817aa68bcd9e2b13a24ba794201ba6dc'],
 ]) {
   run(`generateWorld(${seed}, 512, 384)`);
   check(`seed ${seed} restores corner sealing`,run('CORNER_SEAL')===true);