fix(moq-mux): strip edit lists from the CMAF init

[kixelated]

Problem

The fMP4 exporter copies each source track's edit list (edts/elst) straight into the merged CMAF/HLS init (extract_init). Edit lists are redundant in CMAF -- timing is carried by tfdt and per-sample composition offsets -- and browsers apply them, so a source's initial empty edit (media_time = -1) becomes a real presentation offset.

In practice this showed up as a black screen on VOD/HLS playback in the browser: the applied edit shifted the video track off the audio track's timeline (here by ~2 days, because the empty edit's -1 was being mis-encoded to +2^32-1 upstream), so Media Source Extensions left the video source buffer empty at the playhead while audio played fine. Even with a correctly-encoded -1, an edit list in a fragmented CMAF init is an interop hazard best avoided.

Fix

Drop the edit list in extract_init so the CMAF/HLS init never carries one:

for mut trak in moov.trak {
    trak.tkhd.track_id = track_id;
    trak.edts = None;   // CMAF carries timing via tfdt + composition offsets
    traks.push(trak);
}

This is the only place a source edts enters the output (the synthesized-trak paths don't add one).

Test

extract_init_strips_edit_lists builds a source moov whose trak carries an edts/elst, runs it through extract_init, and asserts the resulting trak has no edts. Full moq-mux suite green.

Note

Blanket-stripping also drops any non-empty trim edit (media_time > 0, used to trim encoder-priming samples). For such sources the first few priming samples would play instead of being trimmed -- a minor start-of-clip artifact, never a black screen. If preserving real trims matters, this could be gated to empty edits only.

🤖 Generated with Claude Code

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 61f8c0e9-d6c0-4291-87c0-dc9303cfcd9f

📥 Commits

Reviewing files that changed from the base of the PR and between 112fa90 and 8c5c0b8.

📒 Files selected for processing (1)

• rs/moq-mux/src/container/fmp4/export.rs

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Walkthrough

CMAF init extraction now removes each track’s edts edit list while rewriting track IDs. A unit test adds an edts/elst structure to an input initialization segment, runs extract_init, and verifies that the resulting track has no edit list.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: stripping edit lists from the CMAF init.
Description check	✅ Passed	The description directly explains the edit-list removal fix, its impact, and the added test.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch claude/strip-cmaf-edit-lists-main

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