feat(alerts): merge signal + checks charts into one "Signal & evaluations" view

[Makisuo]

What

Replaces the two separate, duplicate-looking charts on the alert rule detail page with a single merged AlertSignalChart.

Before, the same metric was drawn twice across two tabs:

• Overview → area chart — the continuous warehouse signal. With a grouped-by-service rule it stacked N translucent filled areas that blended into an unreadable "glittery" mush.
• Checks → sparkline — the engine's discrete evaluations.

Same metric, same threshold, same window, so it read as a duplicate — even though the two sources are genuinely different (raw signal vs. the engine's decisions).

How

One chart, two layers on a shared time axis + one threshold:

• Signal layer — single series → area with a comparator-aware threshold-split fill (--destructive above the line for gt/gte, below for lt/lte, so the fill encodes the breach region); multi-series → thin lines with no fill, which removes the glitter.
• Decision layer — a thin per-bucket evaluation rail of status cells (breached / healthy / skipped), reusing the page's existing sticky-header timeline-strip idiom, with a small legend.
• Incident windows shaded across both via ReferenceArea (matching the existing anomaly-timeseries-chart.tsx pattern).
• raw_query rules — previously showed "preview unavailable"; now render a checks-driven chart + rail.

Per the agreed scope, the checks stat strip + audit table move into Overview and the separate Checks tab is removed (rule detail is now Overview + History). The now-redundant CheckHistorySparkline is deleted.

No new data fetching — chartData, checks, ruleIncidents, and the page time window were all already in scope on the detail page.

Files

• new apps/web/src/components/alerts/alert-signal-chart.tsx
• mod apps/web/src/routes/alerts/$ruleId.tsx
• del apps/web/src/components/alerts/check-history-sparkline.tsx

Verification

• bun typecheck clean across all packages.
• Rendered the component in isolation (mock data, no API) across four scenarios — single-series threshold-split fill, multi-series thin lines (no glitter), raw_query checks-driven fallback, and lt/apdex breach-below fill — all correct, no console errors.

Reviewer notes

• AlertPreviewChart is intentionally kept — it's still used by the create-form live hero (rule-live-chart-hero.tsx), which has no checks/incidents yet.
• The merged chart uses a numeric (scale="time") X-axis so checks/incidents place by real time; warehouse bucket strings and check timestamps are both normalized to UTC ms via normalizeTimestampInput.
• The end-to-end integration on a real rule (warehouse + checks + incidents together) wasn't browser-verified here because the local API/Clerk stack couldn't boot in this environment — worth a quick look on a rule with real incident/check data.

🤖 Generated with Claude Code

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