pondMass/pondCentroid/drainPondAt/dumpPond threw when this.water didn't
exist yet — Lane A's HUD reads pondMass() every frame, including in the
forecast/prep phases before the sail is rigged. Node tests always attach
first so they never hit it; the live game did on the first frame. Guarded
+ a pre-attach assert so it can't regress. Verified in the assembled game:
a flat quad ponds 780 kg (belly sagging below ground), dumps on break,
and the merged browser selftest is 219/0/0.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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