fix(web): improve preview recording frame delivery

[maria-rcks]

preview recordings can lose motion frames when the browser selects av1 at its default bitrate. prefer h.264 with compatible fallbacks, request the selected fps explicitly, and scale the requested bitrate with captured pixels and frames (0.05 bits/pixel/frame, bounded to 2.5–50 mbps). stop media tracks once the encoder has flushed, before allocating the output buffer and saving the file, so native capture does not continue during that work.

verified 34 focused recording/scope tests, scoped lint, and the web typecheck. a repeated 1920×1080 encoder stress benchmark compared the final 6.22 mbps request against a 24.88 mbps reference using the same dense moving pattern for four seconds, reversing run order on the repeat:

four-second recording	24.88 mbps reference	final 6.22 mbps request
encoded frames, two runs	229 / 235	236 / 234
delivered frames per second	57.25–58.75	58.5–59.0
file size	19.52 / 20.31 mb	6.29 / 6.27 mb
moving-pattern psnr, first run	28.03 db	27.37 db

this reduces encoded data to buffer, copy, save, and upload by 68–69% against the high-bitrate reference, with similar frame delivery and a small measured detail loss. total process cpu and peak rss were not measured; bitrate requests are advisory. the earlier upstream av1 baseline delivered 47.8–55.0 fps in two six-second runs of the dense pattern, so final frame delivery remains improved over that baseline, though file size remains larger than av1.

the benchmark uses canvas capture and real browser encoders in the attached mac preview, with native recording active in both cases to keep the background tab unthrottled. the current source's encoder factory was evaluated in that browser and selected h.264 at 6,220,800 bps. the installed desktop client has not been rebuilt, so the changed native recording interaction remains unverified. the uploaded original output decoded locally and played through from its uploaded url at 1920×1080 with 236 frames.

the image below is a reduced 320px / 10 fps motion preview. measurements use the original 1080p recording.

model: gpt-5.6-sol. harness: codex.

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[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This production recording change alters default encoder selection, bitrate policy, capture frame-rate constraints, and stream-release timing for existing recording flows. The scope is focused and tested, but the product-default changes warrant human review.

Not approved because:

• Monthly spending limit reached (workspace setting). Approvability relies on correctness review in order to determine eligibility

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