feat: registry owns correlation discounting (AR-024, AR-025)

Moves two responsibilities inside the registry that callers should never have
been trusted with.

AR-025 — per-frame evidence is discounted for correlation by the registry
itself, via EvidenceDiscounter. Log-odds accumulation is only valid for
independent observations, and consecutive frames of one track are anything but:
near-identical pose, lighting and expression. Accumulated naively, thirty frames
of the same face at the same angle drive the posterior to certainty on what is
effectively one measurement.

Each observation is weighted by how much it adds — a view already contributed
counts for ~nothing, a genuinely new pose counts in full. This reuses the
novelty judgement gallery expansion already makes rather than inventing a second
one. The discounter is a separate class the registry holds, so it stays testable
and swappable, but it is a constructor argument rather than an option: there is
no correct way to accumulate without it.

AR-024 — observe() takes a calibrated probability and converts to log-odds
internally. A caller can no longer hand it a raw cosine, which would have been
silently wrong rather than obviously so. Retiring the remaining raw-cosine
constants in the tracker is still open.

DeadTrack now reports effective_obs alongside observations: the raw count and
the evidence that actually counted. A large gap between them is a track the
camera stared at, and worth seeing.

Three tests, one of which is the point: two tracks given the same number of
observations at the same posterior, one repeating a single view and one seeing
eight distinct ones, must not end up equally confident. Without discounting they
would be identical.

Fixed a test that asserted a belief swap on tied evidence. A tie leaves
ownership where it is — a challenger must out-accumulate the incumbent, since
one contrary observation is noise. The original test passed only because it fed
raw log-odds directly.

Suite: 78 cases, 3245 assertions. Coverage 20/63 to 22/63.

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

TRACES: AR-024, AR-025 | SR-002

docs/requirements.md

@@ -51,8 +51,8 @@ Status: `Done` · `In Progress` · `Planned` · `TBD` · `Withdrawn`
 | AR-021 | Cluster unknown tracks into one entity per person, under temporal cannot-link constraints | SR-005 | Medium | Planned |
 | AR-022 | Capture still-unidentified tracks: embeddings, metadata, **context crops** for human review | §4 | Medium | Planned |
 | AR-023 | Fit sigmoid calibration from intra/inter similarity distributions | SR-002 | High | Done |
-| AR-024 | **Always the calibrated probability, never a raw cosine** — exceptions recorded | SR-002 | High | Planned |
-| AR-025 | Per-track Bayesian accumulation in log-odds, with correlated-observation discounting | SR-002 | High | Planned |
+| AR-024 | **Always the calibrated probability, never a raw cosine** — exceptions recorded | SR-002 | High | **Done** (registry boundary) — `observe()` takes a calibrated probability and converts to log-odds itself; retiring the raw-cosine constants is still open |
+| AR-025 | Per-track Bayesian accumulation in log-odds, with correlated-observation discounting | SR-002 | High | **Done** — log-odds accumulation with correlation discounting owned by the registry, `src/evidence_discount.hpp` |
 | AR-026 | All similarity computed as GEMM, including annex and deferred pass | SR-001 | High | In Progress |
 | AR-027 | Throughput acceptable for **arbitrary** gallery size | SR-001 | High | Planned |