Closes QW2. Validated against the dev box's AMD Raphael iGPU
(/dev/dri/renderD128, radeonsi/mesa 25.2.8). The "proper" full-GPU
path via scale_vaapi triggers a known mesa 25 + Raphael bug
("Cannot allocate memory" per session start, encode still succeeds
but logs are spammy) — hybrid CPU scale → format=nv12 → hwupload
→ h264_vaapi encode delivers GPU surfaces to the encoder without
poking the broken scaler.
Three concrete changes in buildHLSFFmpegArgsAt:
1. New `case "h264_vaapi"` adds `-vaapi_device /dev/dri/renderD128`.
Multi-GPU hosts (this dev box has NVIDIA on renderD129 + AMD on
renderD128) need it so the encoder doesn't bind to a non-VAAPI
render node — without it the encoder fell back to NULL device
in manual smoke testing.
2. Filter chain branches on codec: VAAPI uses
`scale=…,format=nv12,hwupload` while libx264 / NVENC / QSV
keep the existing `scale=…,format=yuv420p,setparams=…` shape.
The setparams color metadata block is dropped on VAAPI because
VAAPI surfaces don't expose VUI fields and the encoder writes
its own.
3. Two new unit tests lock the argv shape so a future refactor
doesn't accidentally merge the paths back together:
TestBuildHLSFFmpegArgsVAAPI asserts the new flags + the
ABSENCE of scale_vaapi; TestBuildHLSFFmpegArgsLibx264NoRegression
verifies the software path keeps yuv420p + setparams + has
none of the VAAPI extras.
Manual ffmpeg validation on the dev box:
hybrid encode of 5 s 4K → 720p: 0.66 s wall, 472 % CPU, 268 KB
output — no errors logged. scale_vaapi variant in comparison
spammed "Cannot allocate memory" while emitting valid output.
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