Store signatures, and age the ones that stop mattering

The spike proved landmark hashing separates a real repeat from a
coincidence; this is the library it unlocks. Rebuilt test-first rather
than lifted from the throwaway harness, against a fixture whose
partials shift several times a second -- a steady tone has only a
handful of peaks repeated endlessly and collides with itself far more
than any recording does, so it would have flattered the design.

Two kinds, which behave nothing alike. A cue is a show's own
furniture, belongs to one feed, and never expires: a break stinger is
not a campaign, and a show that goes quiet for a year comes back using
the same one. It goes when the subscription does. A copy is one
advertisement, belongs to nobody, and is forgotten six months after it
was last heard.

Confidence is a count of separate occasions, and a whole batch of
episodes mirrored in one minute is one occasion however many of them
carried the audio. That is the same independence problem that made
eight agreeing episodes prove nothing about an intro -- they all ran
whatever campaign was running that minute. Nothing is matched against
until it has been seen twice.

Signatures merge by matching rather than by identity, because the same
advertisement re-encoded by another CDN is not byte-identical and
would otherwise pile up as a new signature every week.

Stored as packed base64 rather than JSON arrays: three thousand hashes
per signature is 30KB as numbers and a quarter of that packed, and
this file gets parsed on every start. The index is deliberately not
stored, and is rebuilt on load -- skip that and every signature
silently stops matching after a restart.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

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