feat(dashboard): one notice at a time — the banner priority slot

[kevintseng]

What

UX-6 of the work-topology arc (shrunk: the Analytics fold-in already shipped with the tab consolidation). The Doctor, Onboarding and Insights banners could stack three deep above the nav — three interruptions competing for the same glance. They now share a notice slot:

• Each banner keeps deciding its own eligibility (an ineligible banner renders no DOM node) — no lifted state, no new coupling.
• App orders them by priority: Doctor (broken install) > Onboarding (empty library) > Insights (pending proposals).
• One stylesheet rule (.notice-slot > * ~ * { display: none; }) shows only the slot's first rendered child. display:none keeps the losers out of the accessibility tree as well as the viewport; the next in line surfaces the moment the winner is dismissed or its condition clears.

Verification

• node scripts/run-tests-isolated.mjs → exit 0, 139 files / 2095 tests (net +3).
• npm run verify:release → exit 0.
• New tests/dashboard/notice-slot.test.tsx pins both halves of the mechanism (happy-dom cannot observe CSS cascade, so the rule is pinned at source): the Doctor > Onboarding > Insights DOM order with all three eligible at once, the next-in-line takeover when the doctor banner's dismissal signature matches, and the stylesheet rule itself.
• Break-tests: swapping the App banner order → × renders all eligible banners … in order red; deleting the CSS rule → × the stylesheet hides every slot child after the first red; restored → 3/3 green.