feat(announcements): open the full announcement from the banner text

[philmerrell]

The announcement banner pill renders no body, so its only affordances were a ✕ and an optional external CTA — which trains people to dismiss unread, with What's New (buried in the user menu) as the only other route to the content.

The pill's text is now a button that opens the announcement's own dialog, and the dialog owns the ack from there: any of its exits writes dismissed (or acknowledged) and retires the pill durably, on every device. Having read the body is a stronger signal of consumption than clicking ✕ on a one-line strip, and leaving the pill up afterwards would only ask the user to dismiss something they have already dealt with.

It opens the single-announcement dialog rather than the What's New list deliberately: the pill named one thing, and handing back a list to search through is a worse answer than the thing itself.

Supporting changes

• AnnouncementModalData.sourceSurface (default modal) attributes every ack to where the gesture happened, so reach stats can tell a banner that earned a read from an interruption nobody asked for. No backend change is needed — AnnouncementSurface is a plain 3-value literal and the ack route never cross-checks it against the announcement's own surfaces. The funnel counters key on action alone, so attribution cannot distort them.
• AnnouncementModalService.openFor() is now the one entry point for a user-initiated open, and dialog construction lives in a single private method so openRef cannot drift. It skips the §D8 turn-safety gate — a click is not an interruption, the user is the one asking — but still refuses to stack on an open dialog, and marks the item shown so the auto-modal cannot re-interrupt with something already read. disableClose still tracks requiresAck even when the user opened it themselves.
• A requiresAck announcement gets no ✕ on the pill. This closes a real hole: dismissal suppression is rank-based and covers the banner and modal slots alike, so a ✕ on the strip let a user retire a compliance notice before the blocking modal ever fired — leaving no acknowledged record anywhere. On those, the only way out is to open it and press the button.

Accessibility

The text button uses an sr-only prefix span rather than an aria-label. The visible line may be the summary, so a title-derived label would leave the visible words outside the accessible name — a WCAG 2.5.3 (Label in Name) failure for voice-control users. The untruncated headline stays reachable via title.

Testing

25 new cases across the banner, modal, and modal-service specs, including the ack-attribution matrix, the requiresAck ✕ suppression, and openFor bypassing the gate while still respecting openRef. Full SPA suite: 2510 passed; tsc --noEmit clean.

Not browser-verified — SKIP_AUTH=false locally, so this needs a manual pass. Worth eyeballing: truncation and left-alignment of the text button at ~500px width, the focus ring, and that the banner stays visible behind the dialog until the ack lands.

Spec updated: docs/specs/feature-announcements.md §D1 (banner) and §D2 (the ack's surface records where the gesture happened, not which surface owns the announcement).

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