Replace the userspace lock-file scan + random-jitter retry loop with
Xvfb's own -displayfd mechanism. Xvfb scans up from :0 and atomically
binds the first free X11 socket (kernel-mediated, no userspace race),
then writes the chosen display number back through an inherited pipe.
This eliminates the duplicate-display race that occurred when many
camoufox processes started concurrently and all observed the same set
of free display numbers before any of them bound.
Adds a 10s read timeout on the displayfd pipe so a hung Xvfb fails
fast instead of blocking forever, and adds tests covering single
launch, idempotent get(), 50 concurrent reservations with uniqueness,
and post-kill display reuse.
Jake Writer