The live acceptance question (Mouth flag apparently not surviving
crop) resolved to hypothesis (b): the terminus genuinely lay outside
both test windows — the Lendel mouth chord is ~108.5km, not the ~38km
estimated, so a District n=32 window centered on the cell OR the
midpoint misses the bisected waterline; a seaward-cell-centered
window provably ships Mouth. Hypothesis (a) — upstream-only crops
collapsing the flag — was FALSIFIED directly with a constructed
window (upstream anchor cut, true terminus in range -> Mouth ships;
crop reads only last_in). This test converts that probe into a
permanent pin with construction-sanity asserts, closing the blind
spot where the existing GJ1c acceptance test's bbox-derived window
always contains the whole course. Revert-verified: reintroducing a
first_in==0 requirement fails this test by name while the whole-
course test blindly passes — exactly the gap. Probe file deleted.
cargo test --lib 1877/1877.
Tickets: T-1170
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
jpmschweitzerandClaude Fable 5
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