Distinguish animation and IK target jumps in offboard bail capture

docs/OFFBOARD_FALL_INVESTIGATION.md

@@ -0,0 +1,35 @@
+# Offboard fall investigation
+
+User reproduction: `logs/game-20260906-224637.stderr.log`, default map,
+Hardcore, BipedAir state 501. No automated or agent gameplay tests were run.
+
+The four captured airborne ticks show drive weight 1 and no reported world
+contacts. This does not exclude self contacts: the feedback publisher filters
+same-actor contact reports. Bail reason 18 is requested at tick 4.
+
+The crucial change is in the target, not merely a physical leg drifting away:
+
+| Tick | Part 19 drive position (animation coordinates) | Physical position (world coordinates) |
+| --- | --- | --- |
+| 1 | 0.017, 0.475, -0.596 | 2.643, 0.444, 2.526 |
+| 2 | -0.082, 0.411, -0.573 | 2.603, 0.361, 2.591 |
+| 3 | -0.549, 0.294, -0.335 | 2.394, 0.215, 2.944 |
+| 4 | 0.537, 1.619, -0.047 | 2.642, 0.408, 2.682 |
+
+At tick 4 part 19 has 1.206 m pose error and part 21 has 0.803 m, exceeding
+the existing squash check. Both feet have local IK targets and full external
+blend. Their retained local translation deltas are effectively zero in XYZ.
+The existing capture did not contain the unmodified animation pose, so it
+cannot distinguish an animation target discontinuity from an IK-generated one.
+The next capture includes original joint positions, pre-IK volume positions
+and basis lengths, final foot target frames, and the current animation name.
+
+Static comparisons found no basis to change the air trajectory, disable the
+bail, or change the joint solver. Native foot IK uses a rigid transpose for
+the external adjacent-volume frame (82BEE390), and native prelanding leg
+extension really multiplies the accumulated height (82BAFD30); neither is an
+established defect. MatchAirTime (82BBA070) seeks the current sequence to the
+fraction of elapsed flight. Generated native instruction extracts are in
+`logs/biped-*.txt`; authoritative TU3 sources remain in Skate3Research.
+
+No gameplay fix is claimed by these diagnostic commits.