Adds an opt-in search strategy toggle in the Quality Profile:
- priority (default): unchanged — first source in the hybrid chain that
meets a quality target wins.
- best_quality: pool candidates from EVERY source per query and download
them best→worst by actual audio quality; source order only breaks ties.
Implementation reuses existing plumbing so the retry system is untouched:
- engine.search_all_sources pools raw tracks across all configured,
non-exhausted sources (no first-source short-circuit).
- candidates.order_candidates: new quality_first sort path — profile
quality rank dominates, confidence/peer signals break ties. Priority
path is byte-for-byte unchanged (regression-locked by tests).
- task_worker passes quality_first + targets through; skips the redundant
hybrid-fallback block in best-quality mode (pool already covered it).
- Per-source retry budgets unchanged: a source that spends its budget is
added to exhausted_download_sources and thus dropped from the whole
pool. Independent of post_processing.retry_exhaustive.
- Query generator NOT touched.
Also clarifies the "Allow fallback" setting wording: it accepts OFF-LIST
quality as a last resort (not "walk down my list"), and notes that
lossy_copy.downsample_hires also bypasses the quality gate — the cause of
16-bit/MP3 files slipping through a 24-bit-only profile.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
