perf(test): enforce a per-suite wall-clock budget in CI, warn then fail #10230

Parent: #3716 (slice 3 of 3). Runs in parallel with slice 1.
Runs in: lane.

Problem

#3716 criterion 6: a documented per-suite wall-clock budget that warns above a threshold and fails above a ceiling, so a suite cannot grow past what a contributor can run. Today nothing enforces a per-suite time: scripts/plan-test-lanes.sh uses scripts/suite-seconds.txt only to size and balance CI legs (ceil(seconds / budget)), and fleet-state.test.sh once passed the 600 s tool maximum on its own (#3716 comment). #8757 covers the separate local affected-run budget; this slice is the per-suite CI check.

Fix

1. Add a CI check that reads each suite's measured wall seconds from the test legs of the run and compares it with a documented threshold and ceiling: above the threshold prints a warning naming the suite; above the ceiling fails. Seed the threshold and ceiling from the current scripts/suite-seconds.txt distribution so no suite on main fails on day one; state the numbers and their basis where the check is documented.
2. A suite that legitimately needs more gets a named per-suite exception in one listed file, so an exception is visible in review.
3. Document the budget in the scripts' docs where contributors read how to run tests.

Done when

• A test for the check shows a suite above the threshold produces a warning and exit 0, and a suite above the ceiling exits non-zero
• The check runs on pull requests and is green on main's current suite times
• The threshold, ceiling and their basis are written in one documented place, and the exception list is one file
• ci-status is green on the PR head

Split by the backlog HITL session (2026-10-25, owner rule Q93: split large work before a run picks it up). This was generated by AI.