[zachspartofaday]

Xorg/Present can stall when the virtual CRTC counter advances only on page flips. Separately, desktop virgl workloads need transparent-zero sampler borders and reliable host/guest GL-context switching. This PR fixes those related prerequisites for the virtual Metal GPU path.

• Advance the virtual 60 Hz clock independently, implement legacy/modern vblank waits and events, reserve flip-completion capacity, cancel/join per-file workers on close, use monotonic Linux deadlines, and interrupt synchronous waits through the guest signal protocol.
• Reset the virgl host context before embedder jobs. Add a private ANGLE/Metal zero-border operation that validates its inputs; arbitrary border colors remain unsupported and the full GLES border-color extension stays unadvertised.
• Retain preferred-source patches, licensing and pinned build instructions, plus macOS static qualification support. No guest Mesa binary or SteamOS installer is included.

This consolidates fork #6 and fork #7, whose final heads passed external Codex review before merging into our fork. One focused commit is based directly on main; it does not include storage PR #18 or the separate desktop (#20) and SteamOS (#21) submissions.

Validation on Apple Silicon macOS:

• Fresh ARM64 host interpreter, JIT disabled and real vDSO: expanded DRM contract passes idle progression, synchronous/async events, queue capacity, rejected-flip scanout preservation, 16 close-before-deadline cycles, eight guest signal/EINTR/reap cycles, and existing buffer/PRIME/KMS checks. The fixture uses static musl via Zig and a retained fakefsify tool for fixture import only.
• Current host renderer compiled and passed repeat/edge/zero-border/mirror-edge submissions plus nonzero-border rejection using retained pinned ANGLE/epoxy/virglrenderer dependency builds.
• Shell syntax, Python compilation and source checks pass. The grouped union composes with the other submissions to reproduce the merged fork's product/test/evidence tree exactly.

Limits: the clock is virtual, not panel synchronization. Sampler acceptance alone is not guest pixel or desktop qualification. The guest border-pixel contract is retained but was not rerun during consolidation; Linux wall-clock fault injection, a clean dependency rebuild and a new physical-device run were not performed.

Details: docs/VIRTUAL_DRM_TIMING.md, docs/GPU_SAMPLER_COMPATIBILITY.md, docs/BUILDING.md.