Rebuild lake generation: bowls in valleys filled to their spill level (t96)

Lakes no longer pour water to a guessed lip and repair it afterwards. Each lake lowers the land
in the nearest valley with a smoothstep bowl, and water fills it to its spill level (the lower of
the highest ground on each side, Priority-Flood in one dimension), so it is flat and contained by
construction. Clay beds, no caves or loose pockets close under lakes, trees kept off the shore.
Removes the levelling pass, the lower-rim lip and the bank-slope cap.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Cj2RPXdPykKMEz5BXXczsj

test/t_corner_seal.js

@@ -4,11 +4,11 @@ const { createHash } = require('node:crypto');
 const { run } = load();
 const { check, done } = suite();
 
-// t96 re-baseline: only lake generation (rim height, spawn clearance, levelling) changes these material bytes.
+// t96 re-baseline: only lake generation (bowl, spill level, clay bed, no caves under lakes) changes these bytes.
 for (const [seed, expected] of [
-  [777, 'dc0a19b076f341bcd944e1692651446198f57c7353db5fdea36d6bdd886ab6ff'],
-  [77, 'aa979290d81a758e6ff68fa22b06513fb546aa73a80d09e69b35899a46758a0b'],
-  [4242, '25737574612fe0395a2b0304a1ac698f817aa68bcd9e2b13a24ba794201ba6dc'],
+  [777, '6c75cd15278a0d8c952bfb0b0f7f6185c597bd8f607d112d5970f8bdff4b1f2f'],
+  [77, 'c1f3b0ac9b6d0a566e0051fa3f52388fa626ee714c3e18feed235721569ba056'],
+  [4242, '63f6d90358f3e53be577a724cc5265a6e54380c431196cd3ed24cfd064285924'],
 ]) {
   run(`generateWorld(${seed}, 512, 384)`);
   check(`seed ${seed} restores corner sealing`,run('CORNER_SEAL')===true);