Fix the regression introduced by the weighted Laya signal (#16):
rebuild now merges exact fingerprints in every engine mode after
evaluating hard blockers, compares candidates against all variants
of a group, falls back to deterministic scoring when Laya is
unavailable, and reports transaction errors safely. Non-regression
tests cover the Laya-mode rebuild path; verified against a real
catalog copy (39,341 streams rebuilt successfully).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
6b16d2527d