perf: ship bounded multipart SABR downloads

[Priveetee]

Summary

• replace static SABR segment downloads with framed streams from Server
• download independent ranges concurrently with bounded worker counts
• reserve peak disk capacity before accepting large jobs
• detect stalled streams without imposing a total duration limit
• reduce progress lock contention and reuse transfer buffers

Production behavior

• exact selected audio and video formats remain preserved
• large downloads use up to 12 concurrent parts
• long downloads continue while bytes are arriving
• stalled streams are cancelled and retried
• active reservations prevent concurrent jobs from exhausting local storage

Tests

Downloader CI and Coverage workflows are the release gates. No additional local validation was run for this PR.

Rollback

Restore Downloader revision 7763ee165333a0132a59ec25b4548a4b9084bb16.