R-1: Follow the coasted LOST target briefly while it's still moving

[TCVinNYC]

Reopens #143 (auto-closed when its stacked base merged as #142); identical change, rebased onto main — now just the R-1 commit.

Summary

Worker-side half of predictive tracking (tracker-side coast merged in #142):

• _maybe_track publishes the locked target's LOST track (coasted bbox + decayed velocity); all other LOST tracks stay filtered.
• _drive_ptz_auto follows a lost target iff hypot(vx, vy) >= 1.0 px/frame (_COAST_FOLLOW_MIN_V, module constant). Velocity decay makes it self-limiting: a mover is followed ≲1 s through occlusion then falls back to hold→coast→search; a stationary occluded subject is never followed.
• Exactly one TrackInfo carries the target id during coast (no held-duplicate) — test-pinned.

Evidence

• 7 RED-first tests (tests/test_worker_coast_follow.py); focused suites re-run green after rebase.
• Task review: Approved, 0 Critical/Important findings.
• Headless Mark A/B (posted on Predictive coast: LOST tracks follow damped pre-loss velocity #142/R-1: Follow the coasted LOST target briefly while it's still moving #143): no throughput/drop/id-switch regression; coast improvement quantified by unit tests (re-acquire spike 40→<25 px/frame); GUI/desktop validation session next.

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