feat(alerts): acknowledge incidents from the incident page and the inbox - #2288

🟢 Confidence 10/10 · safe to merge. The delta since 9044aca is one exported counting helper, a badge swap, an Option rewrite and two new scene assertions; the wider ack flow was reviewed and preview-tested.
_{no findings · tests covered · risk medium · 2/2 new units observable}

Adds acknowledge/unacknowledge to the incident page header and the inbox rows, plus an ack-derived timeline, subline and heading counts. The change is safe to merge; the part added since the last review only narrows the Inbox tab badge to unacknowledged incidents.

• inboxTab renders an Acknowledge/Unacknowledge button per row, lifted over rowLink
• unacknowledgedCount now drives the Inbox tab badge instead of the open-incident total
• buildIncidentTimeline merges incident activity events and pins the live state last
• acknowledgeAction adds a header button on the incident page, absent once resolved

_{a8ee351 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}