fix(web): count open anomalies apart from firing incidents on the overview banner - #2360

🟢 Confidence 9/10 · safe to merge. The anomaly split is correct and pinned by a scene test; the all-clear assertion at overview.scene.test.ts:246 still cannot fail.
_{2 notes · tests partial −1 · risk medium}

The Overview banner now separates anomaly-rule incidents (amber "Open") from firing incidents (red "Firing now"), drawn as one slim strip that disappears when nothing is open. The count split is correct; the two findings already open on this pull request still stand.

• heroCountsOf counts anomaly-rule incidents apart from firing ones
• alertFiringHero returns no element when all clear, dropping the All clear card
• New /lab/anomaly-alerts?state=overview-banner state shows each strip case

Still open from earlier reviews

• 🔵 Note · F1 · bg-severity-warn/[0.04] breaks the tone alpha steps in docs/design-rules.md · apps/web/src/page/overview/view-hero.ts:85
• 🔵 Note · F2 · without live alerts banner test cannot fail after the strip change · apps/web/src/page/overview/overview.scene.test.ts:246

_{eaab70e · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}