Add the in-memory coverage gate the search hot path will consult to
decide whether semantic results may be served for a set of item types.
The tracker reads an atomic.Pointer snapshot with no DB and no lock on
the read path, refreshes single-flight under a mutex on a 2-minute
ticker, and applies per-type hysteresis (enable 0.90 / disable 0.80,
holding the previous latch inside the band) to avoid flapping. It
collapses to not-ready when the active embedding model changes so a
stale latch cannot leak across models, and fails safe: a not-yet-
computed snapshot reports not-ready (never panics), and a model-lookup
or count-query error retains the last-good snapshot instead of
publishing zeros.
Refresh counts through the existing catalogSemanticCoverageByType via an
injectable fetch seam, so the behavior is unit-testable with canned
counts and a fake model provider (no pgx.Rows faking). Reuses the
Task-1 coverageQuerier and extends catalogTypeCoverage with Ratio/Ready,
filled by the pure computeCoverageSnapshot helper. Adds the
SemanticCoverageGate interface beside the other provider interfaces;
Task 4 wires the gate and starts the refresher.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
