Task: Refuse a second build in the same worktree

A worktree runs at most one compass build or test run at a time, and a second one says which is running instead of racing it.

• compass build and compass test run take a per-worktree lock before waiting for a host slot, and refuse with a warning naming the holder when another process of the same worktree holds it.
• Builds in different worktrees still share the host slots as before.
• The compass test suite covers both the refusal and the release.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Every number this task claims about a tree, with the command that produced it and the commit it was measured at. A number without both is not a measurement, because a clean-up record describes a tree and trunk moves under a branch: a consumer count, a gate's component count and a suite's size all change with no code changing, and the reader of a bare number cannot tell which tree it came from. Where the raw output matters, put it in an evidence/ file beside the task and name that file in the Command column.

Found on 2026-10-04 while building this story's configuration document operations: a second compass build started while the first, whose make had hit a link error but kept running other jobs, still held slot a; the second took slot b, and both linked the same library in one build tree. The operator asked compass to check and warn. It lives under this story because that is where it surfaced; it concerns compass alone.

Manual QA scenarios (scaffolded via compass add test_scenario) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.

compass build and compass test run now take a per-worktree lock, build/output/compass-build.lock, before they wait for a host slot. When another process of the same worktree holds it, compass prints the holder's pid, command and start time and exits 1, so a second build is refused at once instead of taking the other host slot and racing the first in one build tree. flock releases the lock when the holding process exits, however it exits, and a repeat call in the same process keeps its lock. Builds in different worktrees share the host slots as before.

Tests cover taking the lock, refusing a second holder and naming it, freeing it when the first ends, and keeping it across calls in one process; a live run of compass build against a held lock refused at once.