Level lakes at the end of world generation, keeping their water (t96)

Volume-preserving levelling pass for water bodies of 50+ cells; the dock foundation is placed
first so it cannot erase levelled water. Seed hashes re-baselined (water and air only).
Started by Codex, finished and reviewed by Claude after Codex hit its usage limit.

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

test/t_corner_seal.js

@@ -4,10 +4,11 @@ const { createHash } = require('node:crypto');
 const { run } = load();
 const { check, done } = suite();
 
+// t96 re-baseline: only the generation lake-levelling pass changes these material bytes.
 for (const [seed, expected] of [
-  [777, 'b6f7b493e93a59e38ee57d33f24690daa87e7be0f4b356b32225a3a6f1a9d001'],
-  [77, '5e08c24603686c8617dd1156b10612b91a05fd9ee8b3648dfc4ee32fc008383a'],
-  [4242, 'f7ca75d19eba9eae4894b21b95fe1b683bd54e1f65a415dcc296f3c23b5d7e05'],
+  [777, '1fac6b4b6420f328d7de87c0813e328303271e7605d85ce541e108361aff0d90'],
+  [77, 'de3ec38547585486924f53abb82cb3c0f76e0bd24b75873ddf96599b5886b52a'],
+  [4242, '57cd89b5518c3e342fc1b91243cd068dc2e38a2d42de84a1c2ee1cbf936c2f1a'],
 ]) {
   run(`generateWorld(${seed}, 512, 384)`);
   check(`seed ${seed} restores corner sealing`,run('CORNER_SEAL')===true);