Lakes fill only to their lower rim and keep clear of spawn and the dock (t96)

The lake lip was Math.min(surfY), the highest ground (y grows downward), so water was poured
above the lower rim and, once levelled, ran up to the trade dock on 4 of 12 seeds. The lip is
now the lower rim edge minus one, and each lake stays 80 grains from spawn. Seed hashes and
the shallow-ore hash re-baselined (changes only under lake basins); golden fixtures also
freeze surfY/dirtY/AMB from the previous generator.

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

docs/TESTING.md

@@ -124,7 +124,8 @@ screenshots go to `shots/t72-{retrace,escape,open}.png`.
   joins 2, sky 12/2, lamp 12, magma 12): the old noise had seams in the references too.
   The remaining crops are frozen pre-removal legacy images. The compressed
   `golden-world-640x*.json.gz` fixtures retain the Phase 2 world bytes that produced them,
-  isolating renderer checks from worldgen RNG changes. Inspect the matching
+  isolating renderer checks from worldgen RNG changes; t96 round 3 added the old `surfY`, `dirtY` and `AMB`
+  columns too, because the renderer reads them and lake generation changed them. Inspect the matching
   `shots/golden-*.png` after visible changes.
 - `test/t_gpu.py` checks default limits (four storage textures per stage), dirty-upload
   readback after 240 busy ticks with scrolling and both looks, exact dirty/full image equality,