Tool fix check, 2026-10-24: /tmp via next dev --webpack on the hubos/ia/20261019 worktree, headless Chromium (Playwright, bypassCSP because dev needs eval), shape Horizontal 1920x1080.
Fix: every segment fps=<first clip rate> (30 if unknown), audio aresample=48000 + stereo fltp, 90 s no-progress watchdog.
Before the fix: 30+24 stayed at Merging 0% (test-results.txt). Script: scripts/merge-fps-check.cjs

viewport 1440 | sample-b-720p30.mp4 + sample-d-720p24.mp4 | OK in 17.1s | stream|codec_type=video|width=1920|height=1080|r_frame_rate=30/1 ; stream|codec_type=audio|sample_rate=48000|channels=2|r_frame_rate=0/0 ; format|duration=6.016000
viewport 1440 | sample-e-720p60.mp4 + sample-b-720p30.mp4 | OK in 23.2s | stream|codec_type=video|width=1920|height=1080|r_frame_rate=60/1 ; stream|codec_type=audio|sample_rate=48000|channels=2|r_frame_rate=0/0 ; format|duration=6.016000
viewport 390 | sample-b-720p30.mp4 + sample-d-720p24.mp4 | OK in 13.1s | stream|codec_type=video|width=1920|height=1080|r_frame_rate=30/1 ; stream|codec_type=audio|sample_rate=48000|channels=2|r_frame_rate=0/0 ; format|duration=6.016000
Note: output frame rate = first clip rate (30 for b+d, 60 for e+b); 3 s + 3 s = 6.016 s.