Floor-priority lane lock for shared-host session builds (FIFO, no starvation) #4995

Problem

Session builds on shared hosts (intel lanes S/L, yoga, gx10) queue on plain flock files. flock is not FIFO, so waiters can starve: on 2026-10-09 a critical-path pre-push waited 87 min in lane S behind ~9 waiters while one job held the lock for 2h50m, and floor (release-path) work queued behind session work with no priority.

Proposal: ticket-dir lane wrapper

• /tmp/lanes/<lane>/<class>-<ts>-<pid> ticket files; a wrapper runs the job when its ticket sorts first.
• Class floor sorts before session; FIFO within a class by timestamp; no preemption of a running job.
• Stale tickets are reaped by dead PID. Only a job's own PID is ever killed.
• A job holding a lane over 45 min is flagged in the lane log (no kill).

Acceptance

• RED/GREEN test: with a session ticket queued first and a floor ticket second, floor runs next; with equal class, order is FIFO; a dead-PID ticket is skipped.
• Wrapper sets taskset and nice per lane config; replaces the ad-hoc flock recipes.
• Host config changes (labels, units, caps) are out of scope and go through infra.

Milestone 0.71.1. Only the PR needs a slot.