Add HLS_FMP4_PROTOCOL to SegmentedFileProtocol

[biglittlebigben]

Adds a second segmented file protocol so a request can ask for fragmented MP4
segments instead of MPEG-TS ones:

enum SegmentedFileProtocol {
  DEFAULT_SEGMENTED_FILE_PROTOCOL = 0;
  HLS_PROTOCOL = 1;        // MPEG-TS segments
  HLS_FMP4_PROTOCOL = 2;   // fragmented MP4 segments
}

Purely additive: value 2 is new, and HLS_PROTOCOL keeps its meaning. The
comments are new too, spelling out which container each value selects.

An egress PR consuming this follows; it can't merge until this is released.

🤖 Generated with Claude Code

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Latest commit: ca076ad

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