Add retargeted Skate idle and carving clip replacements

docs/riding-replacements.md

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+# Newer Skate riding poses
+
+Optional `assets/private/custom/riding.json` replaces 33 clip leaves inside the normal native playback tree: three `R_IDLE_HCOM_*` idle/lean clips and thirty `MED`, `ML`, and `LNG_CRV_POSE_*` carving clips. Missing the file restores stock sampling. Invalid files fail with a descriptive load error.
+
+The host substitutes retargeted body channels before the existing blend, reference-pose composition, and stance mirror commands. Original clip timing is mapped by normalized phase onto the replacement samples. The stock trajectory, board/truck/wheel channels and channel weights are preserved. Pushes, tricks, crouching, speed tuck, offboard motion and the graph's controller parameters are unchanged.
+
+Sources from the installed newer Skate build are `c_proto_onb_idle_rolling_slow_neutral_pose`, `fast_neutral_pose`, `fs_pose`, `bs_pose`, `fs_fast_pose` and `bs_fast_pose`, all with the same `c_proto_onb_idle_rolling_` prefix. Neutral idle includes the 241-sample `neutral_addi` motion composed onto its base pose. Carving uses the corresponding frontside/backside poses, with faster poses for MED, a halfway blend for ML and regular poses for LNG. The native ANGLE children retain their frontside/backside ordering. This adapts those clips to Skate 3's graph; it does not reproduce the newer game's runtime graph or physics.
+
+The retarget uses the actual GLB bind skeleton and native bone axes. Both leg chains are fitted to the existing deck foot positions; foot/toe orientation stays matched to the board. The result is converted to local animation samples relative to the stock RIG_TPOSE, so the normal reference composition still applies exactly once.
+
+Extraction, retargeting, numerical bake, previews and reports are outside the project at `C:/Users/Daddy/Documents/skate-animation-extraction/riding`. `retarget_riding.py` is in its parent folder. Run it through Blender from that parent directory, then run `riding/bake_replacements.py` with Python to rebuild the private runtime asset.
+
+Validation: `cargo test -p skate-game riding_replacements -- --include-ignored` with `SKATE3_ASSET_ROOT` set checks all 33 replacements at four phases, original board/trajectory and channel weights, and untouched crouch/push/tuck clips. The real-asset `stock_skater_startup` test exercises the shared physics pipeline. Source and retarget reports, foot fit errors and render captures are retained beside the bake tools.