KB: incline shadow-vanish co-resolved with gait v5 (user-confirmed live)

The shadow report was a desync symptom, not a bias/tilt bug: the ~2-4 unit
depth-bias margin is tight enough that v4's out-of-phase leg pose showing
through could push the flat quad under a slope. No shadow code changed;
both symptoms cleared together. Noted as a diagnostic hint (check
[sync]/BT_SYNC_LOG before touching bias/tilt for this report class).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

context/locomotion.md

@@ -91,6 +91,11 @@ BT 4.10's shadow is the flat `*_tshd.bgf` proxy drawn as blend-pass geometry, po
 a depth-bias (`D3DRS_DEPTHBIAS`, decal-style) stops it z-fighting/vanishing on inclines. Polar maps
 have STUB `*_tshd` (shadow authored-off on snow). Also the visual-ground conform (`btvisgnd.cpp`,
 render-only lift to the visible terrain triangles — presentation only, never touches localOrigin). [T2]
+⚠ **"Shadow vanishes on inclines" can be a GAIT-DESYNC symptom, not a bias/tilt bug.** The 2026-07-09
+recurrence CO-RESOLVED with the gait v5 fix (user-confirmed live; no shadow code changed). The bias
+margin is only ~2-4 world units, so anything that pops the displayed root pose (v4's out-of-phase leg
+channel showing through) can push the flat quad under the slope → vanish [T2 observation; mechanism
+T4]. Before touching bias/tilt for this report, check `[sync]`/`BT_SYNC_LOG` for channel drift first.
 
 ## Key Relationships
 - Detail: `docs/P3_LOCOMOTION.md`. Uses: [[asset-formats]] (SKL/ANI), [[decomp-reference]] (offsets).