First step of ADR-008. internal/transcode answers one question: can this host
hardware-transcode, and with which encoder.
The rule the package exists to enforce is that a device node is not proof. A
node can exist while the driver cannot encode, and that produces the worst
outcome ADR-008 describes: a transcode that starts, looks like it works, and
then cannot keep up. So every candidate runs a real two-frame probe encode and
is dropped unless that encode exits cleanly.
This host is a perfect illustration and the reason the rule is not theoretical:
it has /dev/dri/renderD128 AND Nvidia nodes. The render node exists, but VAAPI
fails to initialise against it ("Failed to initialise VAAPI connection"). A
detection that trusted the node would have picked VAAPI and been wrong. The
probe rejected it and selected NVENC, which was verified independently with
ffmpeg outside this code.
The probe is injected, so every branch is testable with no hardware present,
including the important one: device present, probe fails. Verified red-green -
ignoring the probe result fails TestDetectProbeFails with "got vaapi, want no
capability".
One real bug found while proving it: the probe used testsrc=size=64x48, below
NVENC's minimum frame size, so a host with working NVENC would have reported no
capability at all. Raised to 320x240, still two frames.
Also added internal/transcode to the arch layer map as a layer-1 primitive
beside probe and engine, which the arch rule caught immediately.
Not wired into the engine registry or playback yet - that is the next step.
