feat(errors): expandable rows with per-error Spotlight strip (PR H)

[coccyx]

Summary

PR H of the v0.9.0 faceted-nav thread. Brings the Honeycomb-BubbleUp-style differential view to the Errors page so users can answer "why is this error happening?" without leaving the page or knowing what filters to type.

What changes for the user

• Expandable error rows. Each error group row now has a chevron (▶); clicking the row toggles a Spotlight strip immediately beneath it.

• Per-error Spotlight. Selection KQL is the error's service + operation + status.code == "2" (ERROR). Baseline is the rest of the time window. The differential surfaces what's distinct about the failing spans relative to everything else.

• Click-to-Search. Clicking any value in the Spotlight panel navigates to the Traces page pre-seeded with the error's service, operation, lookback, and the picked attribute filter. Two clicks from "what's this error?" to "show me the matching spans".

• How-to banner above the table explains what clicking does. No mystery-meat affordances.

Engineering

• New reusable <SpotlightSection> wraps the streaming fetch + panel render. PR I (Service Detail) will drop it in without re-implementing the data layer dance.
• Errors page table rows are now React Fragments so the expanded <tr> lives directly under its parent row in the table flow.

Screenshots

Collapsed — chevron affordances + how-to banner:

Expanded — Spotlight for frontend GET /api/products/{productId}:

Eight ranked attributes (rpc.system, http.response.status_code, http.request.method, http.route, rpc.method, messaging.destination.name, messaging.operation, http.method) with per-value sel/base bars and diff %.

Test plan

• `npx tsc --noEmit` — clean
• `npm run lint` — 0 errors
• `npm test` — 101/101 passing
• `npm run deploy` — packed + uploaded + provisioned on staging
• Playwright capture spec succeeds (collapsed + expanded shots)

Validate on staging

1. Open the Errors page
2. Confirm the accent-colored "how to" banner at the top
3. Click any error row — chevron rotates, Spotlight panel streams in below
4. Click any value in the panel — navigates to Traces with service, operation, lookback, and filter pre-set
5. Click the row header again to collapse

What's next

PR I: Spotlight on Service Detail (service-level slice + per-op-anomaly drilldown) using the same <SpotlightSection>. After that, cut v0.9.0.

Session log

`docs/sessions/2026-05-28-errors-spotlight.md`

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