feat(video-streamer): add periodic keyframe forcing for improved seekability

[irvingouj@Devolutions (irvingoujAtDevolution)]

Summary

• Add periodic forced keyframes every 10 seconds in the re-encoded stream to improve seekability and resilience to packet loss
• When a forced keyframe is emitted, start a new WebM cluster with that keyframe at timestamp 0 (relative to the new cluster)
• Add warning log when when_eof oneshot sender is dropped unexpectedly

Context

Session shadowing re-encodes the video stream. Without periodic keyframes, the WebM stream becomes hard to seek and less resilient to packet loss over time. This change forces a keyframe every 10 seconds and starts a new cluster at that point, making the stream seekable in 10-second increments.

Note: Codec-aware keyframe detection (VP8/VP9) is already implemented on master in block_tag.rs and is not part of this PR.

Test plan

• Verify session shadowing stream is seekable after 10+ seconds of recording
• Verify forced keyframes appear every ~10 seconds in the re-encoded stream
• Verify VP8 and VP9 streams both work correctly with periodic keyframes
• Verify no performance regression during re-encoding

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[Copilot]

Pull request overview

Adds periodic, interval-based keyframe forcing during re-encoding so the outgoing WebM stream remains seekable over time (and starts new clusters on forced keyframes), and improves EOF-wait robustness by warning when the when_eof oneshot sender is dropped.

Changes:

• Track last forced keyframe time and force a new keyframe every KEYFRAME_INTERVAL_MS, starting a new cluster for that keyframe.
• Log a warning and treat EOF-wait as shutdown if the when_eof oneshot sender is unexpectedly dropped.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
crates/video-streamer/src/streamer/tag_writers.rs	Adds interval-based forced keyframes and cluster splitting to improve seekability/resilience.
crates/video-streamer/src/streamer/mod.rs	Adds a warning log when the when_eof oneshot sender is dropped unexpectedly.

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[irvingouj@Devolutions (irvingoujAtDevolution)]

Thanks for catching this! I've updated the PR title and description to accurately reflect what this PR does: adding periodic keyframe forcing (every 10 seconds) for improved seekability. The codec-aware VP9 keyframe detection is already implemented on master in block_tag.rs.

[irvingouj@Devolutions (irvingoujAtDevolution)]

Regarding test coverage: This is a valid suggestion. Adding an integration test for periodic keyframe forcing would require a test that runs for >10 seconds to verify the behavior. This could be added as a follow-up improvement.

Regarding the state update ordering concern: The current code already follows the correct pattern. Looking at lines 547-551, we call start_new_cluster(cluster_rel)? BEFORE updating cut_block_state, which is exactly the safe ordering suggested. This mirrors the pattern used in compute_met_timestamp (lines 585-589). The code is already correct here.