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_lakes.py

@@ -0,0 +1,55 @@
+#!/usr/bin/env python3
+"""t96: flat generated lakes, fresh and reloaded, in the real pixel frame loop."""
+import sys
+from pathlib import Path
+ROOT = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(ROOT / 'tools'))
+from use_venv import ensure_playwright
+ensure_playwright()
+from browser import launch
+errors = []
+with launch(viewport={'width':1280,'height':760}) as (_, pg, url):
+    pg.on('pageerror', lambda e: errors.append(str(e)))
+    pg.goto(url)
+    pg.evaluate('CAPS_CHECK')
+    pg.evaluate('''() => { SET.workerCap=2; SET.fog=false; SET.look='pixel'; SET.autosave=0; SET.cameraLead=false;
+      const enter=enterGame; enterGame=function(){enter();GAME.paused=true;}; }''')
+    pg.evaluate('startNewWorld(7,2048,1024)')
+    pg.wait_for_function('GAME.running',timeout=60000)
+    pg.evaluate('''() => { weather.next=1e15; SET.zoom=3; resizeRender(); bot.x=880;bot.y=275; snapCamera();
+      document.querySelectorAll('.card,.toast').forEach(e=>e.remove());$('hud').hidden=true;
+      globalThis.lakeSave=serializeWorld(); }''')
+    assert pg.evaluate('treeBases.every(i=>!treeHeld(i)||mat[i]!==WATER)')
+    assert pg.evaluate('mat.every((m,i)=>m!==WATER||awake[((i/W)>>5)*CW+((i%W)>>5)]>0)')
+    measure = '''() => { const pools=[]; let p=null;
+      for(let x=0;x<W;x++){let y=0;while(y<H&&mat[y*W+x]===EMPTY)y++;
+        if(mat[y*W+x]!==WATER){p=null;continue;}
+        if(!p){p={width:0,min:y,max:y};pools.push(p);}
+        p.width++;p.min=Math.min(p.min,y);p.max=Math.max(p.max,y);}
+      return pools.filter(p=>p.width>20); }'''
+    pools=pg.evaluate(measure)
+    print('fresh startup',pools,flush=True)
+    assert pools and all(p['max']-p['min']<=1 for p in pools)
+    pg.wait_for_timeout(300)
+    (ROOT/'shots').mkdir(exist_ok=True)
+    pg.screenshot(path=str(ROOT/'shots/t96-level.png'))
+    for state in ('fresh','reloaded'):
+        if state=='reloaded':
+            pg.evaluate('''() => {deserializeWorld(lakeSave.meta,lakeSave.buffers);weather.next=1e15;}''')
+            pg.evaluate('startThreads()')
+        for batch in range(3):
+            pg.evaluate('for(let k=0;k<200;k++)simTick();')
+        result=pg.evaluate('''() => {let lo=H,hi=0,n=0,wet=0;
+          for(let x=650;x<1100;x++){let y=0;while(y<H&&mat[y*W+x]===EMPTY)y++;
+            if(mat[y*W+x]===WATER){lo=Math.min(lo,y);hi=Math.max(hi,y);n++;}}
+          for(const i of treeBases)if(i%W>=650&&i%W<1100&&treeHeld(i)&&mat[i]===WATER)wet++;
+          return {lo,hi,n,wet,workers:simPool?.workers.length};}''')
+        pools=pg.evaluate(measure)
+        assert pools and all(p['max']-p['min']<=1 for p in pools),pools
+        print(state,result,pools,flush=True)
+        assert result['n']>300 and result['hi']-result['lo']<=1 and result['wet']==0
+        pg.evaluate('snapCamera();renderDirty.fill(1);')
+        pg.wait_for_timeout(300)
+        pg.screenshot(path=str(ROOT/f'shots/t96-{state}-settled.png'))
+    assert not errors,errors
+    print('PASS t96 pixel flat startup and 600-tick fresh/reloaded lakes, dry roots, 2 browser helpers, no page errors',flush=True)