Add optional captured crouch and treflip body clips

docs/captured-crouch-treflip.md

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+# Captured crouch and treflip experiment
+
+The optional `assets/private/custom/crouch-treflip.json` replaces body samples in
+the regular ollie crouch transition, ollie/360-shuvit anticipation loops, and the
+default-style low/high 360-flip ground/air slots. Removing or renaming that file
+restores stock playback on the next launch. Other styles and nollie remain stock.
+
+The existing graph owns selection, time, blending, mirroring, attributes and
+trajectory. Board and wheel samples are copied unchanged. Reparented hand/foot
+targets are rebuilt relative to the stock board. This is a body-motion experiment,
+not a new board animation or a complete replacement of every crouch variation.
+
+## Capture and timing
+
+Source: `Videos/2026-09-07 18-24-06.mp4`, 348 frames at 60 fps.
+BlendCap's installed `save_mhr_data.py` processed every frame with hand inference;
+347 detections succeeded. Only the final frame was missing, after the trick.
+BVH generation uses the T-pose skeleton, body smoothing 0.10 s, root smoothing
+0.10 s and vertical smoothing 0.025 s. Original source and capture are preserved.
+
+Source frame numbers are one-based:
+
+| Section | Source frames | Runtime slot timing |
+| --- | --- | --- |
+| Crouch down | 161–186 | 15 keys at 30 fps |
+| Crouched idle | Small closed oscillation over 186–189 | 55/45 keys at 30 fps |
+| Treflip ground/pop | 186–201 | 13 keys at 60 fps |
+| Treflip air/catch | 201–235 | 33/28 keys at 60 fps |
+
+The original video has no sustained crouched idle; that loop is synthesized from
+the bottom pose. Foot IK follows stock contact locations in the grounded sections
+and fades back at catch. The treflip uses the stock actor-relative pelvis path so
+the physics jump is not applied twice. Clip exit blends toward the stock pose.
+These event locations are estimates checked against the video and decoded stock
+clips, not ground-truth motion capture. Controller feel still needs playtesting.
+
+Working files and CLI scripts are in
+`Documents/skate-animation-extraction/video-182406/`. The Blender preview contains
+individual clip actions and a combined `PLAY THIS - Crouch, Idle, Treflip` action.
+It uses the game's existing deformation skeleton; finger animation is not part
+of that 35-bone runtime skeleton. The earlier directly skinned BlendCap file is
+unchanged.
+
+Asset SHA-256:
+`e92bc0ebaf34f467be1da5b4cc1541ac23e7c729eac12cec899f7b641f731e98`.
+The private asset is not checked into Git.
+
+## Validation
+
+`cargo test -p skate-game animation_pose::authored_clips --locked -- --ignored`
+checks board/trajectory preservation, stock slot timing, closed idle body loops,
+and rejection of incompatible skeletons/timing. These tests require private data.
+Build with `Build.ps1`. A `--verify` startup capture was also completed; it is a
+startup/render check, not a controller-driven trick playtest.