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_lakes.js

@@ -17,9 +17,9 @@ function surfacePools() {
   })()`);
 }
 for (const [seed, expected] of [
-  [7, '4c4778417e6cf6e9577b831e66111bd374068a14e15642bae304b8a45a68b879'],
-  [42, '907f2ed79b9fda1ae99c5ee55fa19ce85eff8ffdcd7dadac14a2deb42223217d'],
-  [424242, '40080ea6039d0d4247e36fd9a5837d0675100414f198d77419ecf500c7f9d6ad'],
+  [7, '5e230523dc4dda212e7c67ceb7f3b496ff201d826366e1d7c379efd1351d1956'],
+  [42, 'cc3d150a91caac6ab88dfefbd45d85c5bc5c38fc77d7913b9de157647f4b9d7b'],
+  [424242, '1339c90f4f7f1d245c2e21aa4b8d23b21c199b8251392256f6de5ed704e2b396'],
   [2, null],
   [1, null],
   // Before t96 round 3 these lakes ran over their lower rim up to the trade dock.
@@ -36,6 +36,11 @@ for (const [seed, expected] of [
   check(`seed ${seed} conserves water through levelling`, run('mat.reduce((n, m) => n + (m === WATER), 0) === waterBeforeLevel'));
   const initial = surfacePools();
   check(`seed ${seed} starts with flat surface pools`, initial.length > 0 && initial.every(p => p.max - p.min <= 1), JSON.stringify(initial));
+  // Lakes dug to their lower rim once left a cliff on the high side (seed 766733's 15-grain step).
+  check(`seed ${seed} lake banks slope with no cliff`, run(`(() => { const d = machines.find(m => m && m.type === 'dock');
+    const top = x => { let y = 0; while (y < H && (mat[y * W + x] === EMPTY || mat[y * W + x] === WATER)) y++; return y; };
+    return ${JSON.stringify(initial)}.every(p => { for (let x = Math.max(1, p.x0 - 40); x < Math.min(W - 1, p.x1 + 40); x++)
+      if (x < d.x - 2 || x > d.x + 26) { if (Math.abs(top(x + 1) - top(x)) > 3) return false; } return true; }); })()`), JSON.stringify(initial));
   check(`seed ${seed} keeps its lakes clear of the trade dock`, run(`(() => { const d = machines.find(m => m && m.type === 'dock');
     return ${JSON.stringify(initial)}.every(p => p.x1 < d.x - 8 || p.x0 > d.x + 24 + 8); })()`), JSON.stringify(initial));
   check(`seed ${seed} restores pinned generation switches`, run('LSTEP === 8 && LFILM === 4 && SORTP === 0.06 && CORNER_SEAL && POWDER_FAN'));