RiverNetwork gains river_downstream: Vec<u8> (serde-default, parallel
to river_cells): values 0-7 index drainage::D8 (row,col deltas, N/S/E/
W/NE/NW/SE/SW order); sentinels MOUTH=8, EDGE_DRAIN=9, TERMINAL=10
(reserved — the future endorheic-basin hook, Ruling 2c/7b). Captured
in extract_river_network's existing pass (fdir already in scope, one
map, no new grid pass). Pole-edge D8 exits reclassify as EDGE_DRAIN
and leave the mouths list (Ruling 3f); flat-peak interior no-outflow
cells get EDGE_DRAIN too. cascade_golden deliberately re-pinned: GJ1c
mouths 19->3 — sixteen were pole-edge artifacts, exactly Jeroen's
'circles with no sea in sight'; river_cells/attractors counts
unchanged (pure reclassification + additive field). 21/21 drainage
tests incl. mouth-sentinel/mouths-list bijection on the real fixture
and a synthetic pole-draining-grid case.
Tickets: T-1170
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
jpmschweitzerandClaude Fable 5
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