The water arc, carried since Sprint 3. Rain (Lane C's rainMmPerHour x the
exported RAIN_TIME_COMPRESSION, never hardcoded) lands on each node's
horizontal projection, runs down its steepest of 8 neighbours, and pools
where it can't get out. A flat sail's belly is a basin water flows into
and can't climb from; a hypar drains along its saddle ridge to the low
corners and off — so ponding cannot pincer §7, and measured on real yard
quads a flat rig holds 12 kg/m² vs a twisted rig's 1.7.
The 8-neighbour graph is load-bearing: a 4-way one can't follow the
saddle's diagonal ridge, so it trapped water in the gravity belly and a
hypar pooled as much as a flat sail. Steepest-GRADIENT descent (not
steepest drop) because a diagonal is √2 farther.
Gate 1, in asserts: a flat carport rig under a capped wind survives dry
(4/4) and dies wet (a corner at t=85s) — the control isolates water from
wind — and the broom saves it. Plus dump-on-break, dump-on-tension-up
(the turnbuckle counter-play), mass conservation, and a belly-tear safety
valve at 4 m of sag.
API for D and A, frozen in contracts.js: pondMass(), pondCentroid(),
drainPondAt(node, dt) -> kg-on-your-head. session.reset() for A's "play
again".
On D's tn-1.04 cliff: investigating it IS what surfaced the ponding load
regime. The 10 kN "spike" is real physics, not solver divergence — a
155 m² flat sail holding 2100 kg of water genuinely pulls ~21 kN, stays
finite, and tracks the water. So no physics-altering clamp (the
displacement clamp I tried moved the thesis 39->34%); instead an opt-in
`watchDivergence` tripwire that throws with a repro above 80 kN, live in
every selftest rig and never false-tripping, and a belly-tear that bounds
the runaway physically. Full writeup for D in THREADS.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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