Spike: landmark hashing passes the gate for the signature library

The plan made this the gate: a linear scan of N stored signatures
against each episode does not survive, so the library is only possible
if an index can shortlist candidates cheaply. It can.

Built as a throwaway harness against the live corpus. The episodes the
plan named had rotated out of the server's 8-episode window, so the
test set was rebuilt from current audio and came out stronger than the
one it replaces -- 6 true repeats and 16 non-repeats, against 2 and 3.

Six repeats scored 813-1564 votes; sixteen non-repeats scored 27-94.
The narrowest separation is 8.6x, where the correlator it replaces
manages 4.9x on the same audio. Nothing sits in the gap.

Offsets land within 1.3s and every proposal is cheap to confirm by
correlating against a short window at the claimed position, so the
shape is: index proposes, correlator disposes. That is not a
formality. In one episode the advertisement occurs twice and the index
picked the weaker instance, which only a confirming pass would catch.

Measured: 9.8ms to hash a signature when it is stored, 196ms to look
one up against 23 episodes, ~8s per episode to index, 353MB for 23.
Five hundred signatures cost 98 seconds where linear correlation costs
about 63 minutes.

The harness is not committed -- it read absolute server paths and had
no tests, and its output was an answer rather than code to keep. The
parameters it found are written into the plan so it can be rebuilt
test-first, along with the one weakness worth knowing: the hashes
collide heavily, and that density is what makes a lookup cost
milliseconds rather than microseconds.

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

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