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>

docs/TESTING.md

@@ -76,9 +76,14 @@ The conveyor fixture uses a wider feed pile, the screen fixture warms longer, an
 checks waking within eight ticks separately from the probabilistic consumption of fuel.
 
 `test/t_lakes.js` pins SHA-256 over raw `mat` followed by Float32 `temp` bytes for seeds
-7, 42 and 424242 at 2048x1024. These hashes predate t96 and must not be re-baselined for a
-wake/background-tree change. It also checks generated tree bases, startup wakes, and the
-seed-7 lake leveling within two grains after 6,000 undisturbed ticks, fresh and reloaded.
+7, 42 and 424242 at 2048x1024. t96 round 2 re-baselines only for the lake-levelling pass;
+`t_corner_seal.js` also pins three smaller worlds with the same intentional change. The lake
+suite checks all five seeds (also 2 and 1): surface pools wider than 20 grains start within
+one grain of level and stay so after 600 ticks, exact WATER count from the pre-pass 0.99
+progress callback, dry tree bases and startup wakes. Small fixtures cover solid/powder rims,
+volume, centered partial rows, the 50-grain cutoff and diagonal separation.
+`python3 test/t_lakes.py` checks pixel startup and fresh/reloaded lakes after 600 ticks with
+two browser helpers and no page errors; `shots/t96-level.png` shows the untouched new lake.
 
 `test/t_bot_rescue.js` covers deep basalt burial, nearest/upward and diagonal nudges,
 unexplored-pocket rejection, newest open trail points, ring wrap/load reset, bounded searches,