feat(walk): a -walk harness flag, and the honest reason the stride shot is not in

Screenshots 6 and 7 are approved, so the remaining 0.2.0 work is the traversal
release check. This adds the piece of harness it needs - `-walk` holds forward input
from the first frame, so the pawn actually crosses the sidewalk while the third-person
rig follows it - and records what that check still lacks.

A stride shot was attempted three times and removed: a fixed review camera cannot
catch a pawn moving at 4.5 m/s. Each attempt framed it at a different distance and
the last two missed it entirely, so shipping it would have been a broken shot with a
green verification. The TODO item now says the blocker is the capture harness (a
camera that tracks the pawn), not the game, and names where the shot goes back in.

The flag stays because it is what the traversal check will drive.

TODO.md

@@ -32,7 +32,7 @@ Actionable task list for this project. This file is the single, central place fo
 
 ### Release checks
 
-- [ ] Third-person traversal works end-to-end in the demo area.
+- [ ] Third-person traversal works end-to-end in the demo area. **Blocked on a capture harness, not on the game:** walking works (`-walk` holds forward input; the pawn crosses the sidewalk while the third-person rig follows), but a fixed review camera cannot catch a pawn moving at 4.5 m/s — three attempts framed it at a different distance each run and the last two missed it entirely. The stride shot was removed rather than shipped broken. Needs a camera that tracks the pawn (attach the shot camera to the pawn, or drive the capture from a recorded path), then re-add `demo-third-person-walk`.
 - [ ] No blocking animation or rig defects.
 - [ ] Performance target still met at 1440p.
 - [ ] Tag `v0.2.0` and write `CHANGELOG.md` entry; then remove this section.