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

docs/PROJECT_MAP.md

@@ -378,12 +378,12 @@ The same chunk schedule runs inline or in workers. Workers only own a pass at a
 simTick: one simulation step, in order.
 - functions: simTick
 
-### src/sim/worldgen-helpers.js (131 lines)
-World generation helpers: noise and removal of tiny hard rock specks.
-- functions: makeNoise, fbm, clearHardSpecks, levelWorldWater
+### src/sim/worldgen-helpers.js (74 lines)
+World generation helpers: noise, hard rock specks and lake spill levels.
+- functions: makeNoise, fbm, clearHardSpecks, spillLevels
 - variables: SPECK_MIN
 
-### src/sim/worldgen.js (305 lines)
+### src/sim/worldgen.js (309 lines)
 World generation: staged terrain, ore, caves and placement, with synchronous and async entry points.
 - functions: generateWorld, generateWorldAsync
 
@@ -664,8 +664,7 @@ The frame loop and boot.
 - `test/t_gpu_light.py`: The former CPU light behavioural checks, now read from the actual GPU light grid.
 - `test/t_import_save.js`: Import must preserve the running world before reading or replacing it.
 - `test/t_joins.js`: The oracle states the geometric rule independently of the precomputed neighbour tables.
-- `test/t_lakes.py`: Pixel startup and fresh/reloaded flat lakes after 600 ticks; shots/t96-level.png.
-- `test/t_lakes.js`: Generated lakes start flat, conserve water, and have dry tree bases.
+- `test/t_lakes.js`: Generated lakes sit in a smooth bowl filled to its spill level: flat, contained, clay-bedded, stable, with natural banks, dry tree bases and the dock clear.
 - `test/t_lakes.py`: t96: flat generated lakes, fresh and reloaded, in the real pixel frame loop.
 - `test/t_load.js`: Reload wakes nearby areas first, without losing distant water, fire or saved state.
 - `test/t_molds.js`: Molds and furnaces stay usable: ingots drop below and keep their stubs, plugs over molds are cleared, spoiled castings can be knocked out, grains never drop through a grate by teleporting, and slag lets the fire breathe.