Lane B / session-5 item 4: treaty amendment — required root_space sidecar field

motion.json sidecar gains REQUIRED root_space in {camera, world}. HSMR is
per-frame -> emits camera; Lane C reads it and derives a world root from foot
contacts for locomotion clips. PLAN treaty + Lane C task updated. v1 quality bar:
in-place clips first-class, traveling best-effort until a grounding pass exists.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

PLAN.md

@@ -51,8 +51,11 @@ critical path. Lanes E–F are follow-ons. Do NOT start E/F before D ships.
    - Skeleton: **mixamorig 65-joint** as defined by `~/Documents/character_kit`
      (`female_game.glb` / male base). Canonical bone list generated in Lane 0.
    - Intermediate motion format: **BVH at 30fps** + sidecar JSON
-     (`{fps, joint_order, root_trajectory, source_video, model, version}`).
-     BVH because Blender imports it natively and it's human-diffable.
+     (`{fps, joint_order, root_space, root_trajectory, source_video, model, version}`).
+     **`root_space` ∈ {`"camera"`, `"world"`} is REQUIRED** (session-5 amendment): HSMR is a
+     per-frame recoverer so it emits `"camera"`; Lane C reads this and, for locomotion clips,
+     derives a world root from foot contacts. BVH because Blender imports it natively and it's
+     human-diffable.
    - Units metres, Y-up, character faces −Z (three.js convention; Unreal yaw-fix happens
      at UE import, as established in the A_3D_STORE experiments).
    - Final artifact: FBX (anim-only, Mixamo-style "Without Skin") dropped into
@@ -157,8 +160,11 @@ Tasks:
      `mixamorig_` variants — this bit us before), bake to keyframes.
    - Foot-contact fix: detect frames where foot velocity ≈ 0 & height < threshold → IK
      pin. `# ponytail: threshold heuristic, upgrade to learned contact detection in Lane E if it annoys`.
-   - Trim the T-pose bookends (Lane A protocol guarantees them), root-motion option
-     (in-place vs traveling) as a flag.
+   - Trim the T-pose bookends (Lane A protocol guarantees them). Root motion is driven by the
+     sidecar **`root_space`** (session-5 treaty): `"camera"` (HSMR) → in-place by default, or
+     derive a world root from foot contacts for traveling clips; a `--in-place`/`--traveling`
+     flag overrides. v1 quality bar: **in-place clips are first-class; traveling is best-effort**
+     until a proper grounding pass exists.
    - Export anim-only FBX (Mixamo "Without Skin" style — merge_anims.py already eats these).
 2. QC render: 3-second turntable mp4 of the clip on the female_game body, written next
    to the FBX. **Verify with own eyes** (the rule) — every clip gets its mp4 looked at