docs: replace phased plan with a per-requirement one

The phase structure encoded ordering assumptions that stopped being true as the
design changed, and its Phase 2 still described retuning constants that are now
withdrawn. Ordering is now derived from per-requirement dependencies instead:
anything with no unmet dependency is startable.

Carries over the TrackRegistry design (now keyed to AR-012/AR-013) and records
what was withdrawn from the old plan, including the --presence-mode flag —
comparison against old behaviour uses recorded reference output rather than a
second live code path.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/SPEC.md

@@ -314,7 +314,7 @@ actor was first *recognised*, not when their track began.
 
 **Gap — this is the main behavioural change in this spec.**
 
-Design is settled in [`IMPLEMENTATION-PLAN.md`](IMPLEMENTATION-PLAN.md) §1.1: a
+Design is settled in [`plan.md`](plan.md) under AR-012/AR-013: a
 `TrackRegistry`, held by `shared_ptr` and used by `FaceTrackerFunc` as its state,
 where each track carries `first_seen` plus an **optional `last_seen`** — unset
 while on screen, set to the last on-screen timestamp when the face is lost, unset