refactor(registry): only keep identified tracks associable while dormant

candidates() now excludes dormant (off-screen) tracks that were never
identified: an unowned dormant track has no actor to re-attach to, so
keeping it in the association pool only enlarges the matcher's per-frame
comparison set and invites a new face re-associating onto an anonymous
stub. On-screen tracks are always candidates; a dormant track must have
crossed ownership (t.actor set) to stay associable within
track_extinction_sec.

Measured trade: a small recall cost (~1-3pp on some films, e.g. Lord of
War -2.5) for a cleaner, bounded pool. Kept deliberately. Note it does NOT
fix the ROCm GEMM wedge (that is an intermittent driver flake, not a
pool-size problem) — this is a correctness/cleanliness change, not a
performance one.

src/track_registry.hpp

@@ -218,9 +218,21 @@ public:
             const double clock =
                 reg_.awaits_evidence_ ? reg_.evidence_through_ : reg_.now_;
             for (auto& [id, t] : reg_.tracks_) {
-                if (t.last_seen &&
-                    (clock - *t.last_seen) > reg_.cfg_.track_extinction_sec)
-                    continue;   // retired from association; still awaiting evidence
+                // On-screen tracks are always candidates (actively tracked this
+                // frame). A dormant (off-screen) track is only worth keeping alive
+                // for re-association if it was actually IDENTIFIED: an unowned
+                // dormant track has no actor to re-attach to, so holding it in the
+                // pool only bloats the matcher's per-frame comparison set (every
+                // candidate is a GEMM row) and invites a new face re-associating
+                // onto an anonymous stub. Gating dormant tracks on t.actor keeps
+                // the pool bounded regardless of how large track_extinction_sec is
+                // — which is what makes a long re-association window affordable.
+                if (t.last_seen) {   // dormant
+                    if (!t.actor.has_value())
+                        continue;    // never identified: not worth re-associating
+                    if ((clock - *t.last_seen) > reg_.cfg_.track_extinction_sec)
+                        continue;    // past the re-association horizon
+                }
                 out.push_back(&t);
             }
             return out;