fix: admit HLS prefetches before URL gating

[tcconnally]

Summary

• Admit automatic HLS prefetches before URL construction.
• Return over-budget heavy candidates to the playlist instead of demoting them to direct prefetches.
• Preserve current and queued HLS transcode accounting and forced-transcode behavior.

Verification

• Independent exact-head review: PASS
• Reliability: 54 passed
• Playlist: 10 passed
• macOS playback contracts: 4 passed
• Full tests/run_all.py: overall PASS
• Source compilation and diff check: passed

Corrective follow-up to the previously merged transcode-concurrency fixes.