Role-table version 2: bind the upper chest by the model's chain (MOT-O12)

[snkmcb]

Stacked on #24. It opens against that branch, so this diff shows only role-table version 2. Merge #24 first.

Summary

This PR resolves MOT-O12. In both local models that have 上半身3, the chain is 上半身 → 上半身3 → 上半身2 → 首. Role-table version 1 bound 上半身2 as chest and 上半身3 as upperChest, which is the opposite order.

Ordering the two bones by the chain is not enough. The shared retarget drops a joint the target does not bind, rather than folding it into its child (usd-motion-plugins RETARGETING_POLICY §4.1, case 6). On a target without 上半身3, both orders leave the arms up to 25° off:

• under version 1, the neck carries 上半身2's key twice;
• with only the order changed, 上半身2's key is dropped.

Version 2 (kRoleTableVersion = 2) binds the two bones differently on each side:

• As a target: 上半身2 and 上半身3 bind chest and upperChest in the model's own chain order. Where 上半身3 is off the neck's chain, it binds nothing.
• As a source: upperChest is never emitted. chest is the bone the neck hangs from, and its evaluated world rotation already holds every torso rotation below it. This is the second role, after hips, that a source and a target bind differently.

Measured over 17 local characters (motion C, PMX onto PMX, identity rests; median / largest p95 / max):

Segment	Version 1	Version 2
neck	2.73° / 19.37° / 22.37°	2.66° / 11.89° / 11.89°
upper arm	2.63° / 18.49° / 25.20°	2.45° / 5.26° / 5.26°
lower arm	2.04° / 18.94° / 25.68°	1.82° / 5.62° / 5.62°

Motion C's figures now equal motion A's.

Changes

• mmdSkeletonAdapter: OrderUpperBody sets the target order and SourceUpperBody builds the source roles; the version is bumped. Unit tests cover the conventional order, the inserted order and the off-chain case.
• check_installed_consumer.py: now expects roleTable=2.
• MOTION_CONTRACT:
  • §12.2 has new rows and an "upper chest" paragraph. Its heading drops "version 1", so the links to its anchor are updated.
  • §12.5 now states version 2.
  • MOT-O12 moves to the resolved questions.
• Docs:
  • a new report, docs/reports/2026-09-25-phase9-upper-chest.md;
  • a CHANGELOG entry under "Changed";
  • updates to the roadmap, CAPABILITY_MATRIX and WORKSPACE.

Test plan

• ctest --preset windows-release: 32 of 32 pass, including workspace_installed_consumer with roleTable=2.
• python scripts/check_docs.py passes.
• The before/after figures above come from local runs with the installed build.

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