fix: belief accumulates across frames (lazy-OR), not once

A track recognised on 318 of 385 frames was owned on none, so the truth file
named nobody while the matcher was accepting almost continuously.

The correlation discount was an annihilator rather than an attenuator. Weight
was 1 - P(same view), so once a track had one stored view every later frame of
that same face scored ~0.01 and the belief stopped moving. One observation just
over the accept threshold is logit(0.78) ~ 1.27, under the ownership bar — hence
recognised always, owned never.

Two changes, in the order they were found.

Correlated evidence is now attenuated by effective sample size,
n_eff = n / (1 + (n-1)·rho), each frame contributing the marginal gain. That has
the right shape at both ends: uncorrelated evidence accumulates linearly, and a
held pose converges on 1/rho rather than growing without bound. A constant floor
was tried first and rejected — it grows linearly forever, so a long shot could
out-argue genuinely varied evidence purely by lasting longer.

Combination is now weighted lazy-OR: P = 1 - (1-P_old)·(1-p)^w, stored as
log(1-P) so the update is additive and precision stays where it matters as P
approaches 1. Each frame is new evidence that this track is that actor, and the
belief is the probability that at least one sighting was right. It converges
faster than summing log-odds at the same effective count — 2.98 vs 2.53 after
two observations at p=0.78 — which is what a real clip needs.

Note that summing log-odds was already a correct sequential Bayesian update:
the matcher fits with prior 0.5, so logit(p) IS the per-frame log-likelihood
ratio and the running sum carries the prior forward. It was not wrong, it was
slow. What blocked ownership was the discount, not the combination rule.

Also fixes a real correctness bug: the observation count lived on the
discounter, which is shared by every track, so tracks pooled into one effective
sample and each was discounted by how many others happened to be on screen. It
is now a per-track parameter.

The registry's frame scope holds its lock for its lifetime and the mutex is not
recursive, so calling observe() inside a scope self-deadlocks. The pipeline
never does — separate nodes — but the test did, and hung rather than failing.
Documented at the call site.

Verified end to end: the same clip that produced zero actors now identifies
Bing Crosby and Dorothy Lamour with belief 0.97.

Suite: 96 cases, 6142 assertions.

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

TRACES: AR-025 | SR-002

src/nodes/camera_position_change_detector_node.hpp

@@ -26,7 +26,8 @@
 // The node is a pure pass-through: it forwards the Frame unchanged except for
 // is_cut, so it slots between frame_source and face_detector without altering the
 // downstream contract. eof frames are forwarded immediately without processing.
-
+//
+/// TRACES: AR-009 | SR-002
 struct CameraPositionChangeDetectorFunc {
     static constexpr std::string_view label() { return "camera_position_change_detector"; }
 

src/nodes/embedder_node.hpp

@@ -17,7 +17,8 @@
 // All crops in one frame are batched into a single forward pass (capped at
 // embed_batch_size). The backend serialises itself; we only call it from the
 // single embedder thread.
-
+//
+/// TRACES: AR-006 | SR-002
 struct EmbedderFunc {
     static constexpr std::string_view label() { return "embedder"; }
 

src/nodes/result_sink_node.hpp

@@ -46,7 +46,7 @@ using json = nlohmann::json;
 struct ResultSinkFunc {
     static constexpr std::string_view label() { return "result_sink"; }
 
-    /// TRACES: AR-012, AR-017, IR-002 | SR-002, SR-003
+    /// TRACES: AR-012, AR-017 | IR-002 | SR-002, SR-003
     /// A finished presence claim from the registry. Called from inside the
     /// registry's reap while it holds its own lock, so this must stay a cheap
     /// push and must never re-enter the registry.
@@ -158,7 +158,7 @@ private:
     struct ActorMeta { std::string name, imdb_id, tmdb_id, jellyfin_id; };
 
     // Core logic: merge per-frame detections into annealed [start, end] windows.
-    /// TRACES: AR-012, IR-002 | SR-002
+    /// TRACES: AR-012 | IR-002 | SR-002
     /// A claim already IS a window — `[first_seen, last_seen]` of a track the
     /// actor owned. There is no annealing pass: `anneal_sec` existed to bridge
     /// gaps between isolated accepted frames, and a track that survives its own