D's S16 poison poke, filed for this lane: _checkFailure's corner loop is the
heal's only trigger and it skips broken corners, so once all four were
divergence-broken a SECOND poison had no unbroken corner left to read NaN
through — the corpse stayed NaN forever and rendered as nothing. A lost sail
is still cloth in the yard; it must LOOK lost, not vanish.
The sentinel runs ONLY on a fully-broken rig (the corner wire covers every
other case): sum pos+prev — any NaN/±Inf makes the sum non-finite (Inf−Inf is
NaN), finite cloth cannot overflow it. Branch-free O(n) adds, no mutation, so
a finite lost sail is untouched byte-for-byte.
Test replays D's gait: cascade 4/4 → 5 s finite with ZERO heals (the negative
control — the sentinel must never mistake 'lost' for 'sick') → second poison
heals in one substep, finite a full second on. Node suite 47/47.
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