UX-8 layerStack: add an integration test wired through the real useLayer
hook (popover document-listener + stamp) and a PlayerModal-shaped window
listener, driven by a real dispatched keydown. The popover close is wrapped
in flushSync so its layer is popped synchronously between the two listeners
-- the production mid-dispatch race -- making the escapeHandled stamp the
only thing that keeps the player from also closing. Mutation-checked:
removing the guard fails exactly this case.
SavedViews: cover feedCanSeedDefault's "yield to a bookmarked filter URL"
branch (a filter-bearing search string -> no seed) and the per-source
seededRef guard against the 60s refetch (a changed-list refetch re-runs the
seed effect; the guard blocks re-seed). The refetch test waits for the
changed list to reach the component so the effect has its chance to wrongly
re-fire. Both mutation-checked.
Frontend gate green: 464 vitest (+4). Backend untouched.
