Fix segment pre-caching to progressively cache all playlist segments

[Copilot]

Segment pre-caching was only keeping 1-2 segments cached at a time instead of progressively filling the cache with all remaining playlist segments.

Root Cause

cache_next_segment() only cached one segment per invocation. Since it's only called when a cached segment is consumed or a new playlist arrives, the cache filled extremely slowly—leaving most segments uncached.

Changes

• Batch caching: cache_next_segment() now caches 2 segments per call instead of 1
• Named constant: Added SEGMENT_CACHE_BATCH_SIZE for maintainability
• Progress logging: Added tracking of queue depth and cache progress

Behavior

With 8 uncached segments and segment_drop=6:

Playlist extracted → cache 2, queue: 6
Segment 1 consumed → cache 2, queue: 4  
Segment 2 consumed → cache 2, queue: 2
Segment 3 consumed → cache 2, queue: 0

Cache progressively fills without blocking client requests.

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[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

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I tried to connect to the following addresses, but was blocked by firewall rules:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https /usr/lib/apt/methods/https (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)