diag(track-a): compare default and explicit-GLX Xvfb

[blakinio]

Track A RUNTIME-INFRA — isolated Xvfb GLX enable discriminator

Task: OTC-20260816-track-a-xvfb-glx-enable-probe.
Base: exact trusted main@d3f186414256151c9d5e03f34c5a9026b1fba500.
Execution: exactly one governance-compliant ephemeral_isolated support-process workflow on synology-otclient-01.

Input: Draft #416 proved the exact contained Xvfb has GLX server code/options, contained libGL.so.1, libglx.so and libglamoregl.so, while Draft #415 observed that a task-owned default Xvfb display did not advertise GLX. This task uses those results only to select one support-only differential; it does not promote them.

The workflow starts no official client, VNC or WARP. Within one task-owned namespace it starts a default contained Xvfb, queries GLX/RENDER via core X11, tears it down, then starts the identical Xvfb environment with exactly +extension GLX, queries the same extension state, captures bounded GLX/provider stderr, and tears it down.

No canonical lease/registration/session access, credentials, login, gameplay, Track B access or historical #303 surface access is authorized. No client backend is forced. Exactly one semantic physical workflow run is allowed after valid admission.

Research output remains Draft-only; coordinator promotion is separate.

[blakinio]

Coordinator disposition: CLOSED_SUPERSEDED / negative differential retained. This Draft's default-vs-+extension GLX experiment was a useful intermediate discriminator, but #421 later proved the stronger minimal result: the canonical Xvfb argument list needs no new GLX flag when the contained DRI provider path is supplied. That result is promoted by #427. Keep this closed PR as negative-history evidence; do not merge stale task state.