#11303 removed the O(sessions x roots) path normalization from the session
worktree map, but nothing fails if that shape comes back. Adds the two
checks that were missing, plus the tool that would have caught it.
- timing guard: 1200 worktrees x 400 sessions must build in <150ms.
Verified fail-first — restoring the pre-#11303 per-session
buildWorktreeCandidates call takes it to 238ms; it is ~15ms as merged.
- path boundary: '/repo/alpha-sibling' must not be attributed to
'/repo/alpha'. Hoisting the root normalization out of the loop must not
degrade containment into a bare startsWith.
Also adds tools/benchmarks/workspace-switch-paint-latency.mjs, which
attaches over CDP and measures first-paint-after-click and max frame gap.
The existing worktree-switch-responsiveness.spec.ts only times the
synchronous click task, which stays ~1ms because the highlight is a direct
DOM mutation — that is why a ~1s stall could ship without tripping a
budget. On the affected build it read maxFrameGap p50=973ms.
c5102e1262