release: v0.2.1 — rig fixes changelog entry, TODO pruning

Writes the 0.2.1 entry (fingers, derived arm angle, base keys) and removes the
section from TODO.md. The stride-capture regression is listed under known
limitations rather than omitted: the shot is retired, not shipped.

TODO.md

@@ -9,16 +9,6 @@ Actionable task list for this project. This file is the single, central place fo
 - When all steps in a version section are done: write a `CHANGELOG.md` entry, then remove the version section from this file.
 - Release gate: a version is tagged only on the owner's explicit go-ahead once its release checks pass, not automatically when the checkboxes close.
 
-## 0.2.1 — Rig fixes (patch)
-
-### Patch tasks - active - session-5598ce7e-27e3-4c64-bb49-c04a4a31ee58 (branch `anim-body-0.2.0`)
-
-- [x] Fix skeletal/rigging defects reported after `0.2.0`. Both known defects fixed 2026-11-09:
-  - **Fingers no longer splay.** The bind is an A-pose hand, so the spread carried into every pose. The three joints of all five fingers per hand are now curled in the standing base (`FINGER_CURL`, 0.28/0.34/0.22 rad); the curl axis was probed on the rig first. Fingers 1–5 on both hands, 30 bones in total.
-  - **The arms-down rotation is derived, not swept.** `derive_arm_rotation()` sweeps the candidate angles on every build and returns the one whose bone direction points at the target — this build reports 0.60 rad with a direction dot of 1.000. The hard-coded 0.55 is gone.
-  - Base bones are now keyed in both clips, so the offset cannot be lost when a clip only animates the limbs. **Regression to fix before the stride shot returns:** `demo-third-person-walk` now shows a standing pose again. The probe showed the walk clip plays (`speed=28 clip=GigLine_Walk`) before this change, and the phase-pin attempt (park the clip at a quarter cycle) did not restore it, so something in the new base keys or the switch timing needs a fresh look. The shot is retired rather than shipped broken.
-- [ ] Tag `v0.2.1` and write `CHANGELOG.md` entry; then remove this section.
-
 ## 0.2.2 — Skin and hair tuning (patch)
 
 ### Patch tasks