[defer] feat(dashboard): add per-exit failure breakdown to pool observation

[maxmad64bis]

⚠️ base-red inherited: #14547

Summary

Pool members that fail requests are invisible as a group: the pool screen shows per-member egress traffic but nothing about which exits fail and why, so an operator cannot tell a refusing exit from a quiet one. The egress-observation endpoint now also returns a per-exit failure breakdown (grouped by egress IP and call-log error family, with an explicit unattributed bucket for rows without correlation), and the pool screen shows a 24 h summary card plus per-exit detail. The derived dedup table is wrapped so SQLite never merges unknown into server_error.

Related Issues

No linked issue — pool failure attribution is a new observability surface with no prior ticket.

Validation

• Change type: dashboard
• Focused tests and category gates from the golden path
• npm run lint
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include a new or updated automated test in this PR

Tests Added Or Updated

• tests/unit/pool-failure-breakdown.test.ts (new, 18 tests with oracle): scalar-subquery form wrapped in a derived table — the unwrapped form merges unknown into server_error under SQLite (red 1/2, verified); real base 43k/101k rows: no more MATERIALIZE, 40→20 ms, same rows; 18/18 green on the head.
• tests/unit/ui/PoolEgressObservation.test.tsx (7 tests): summary card + per-exit detail render; 7/7 green.

Coverage Notes

• New query branches (proxyLogs.ts breakdown) and the summary/detail UI hunks are covered by the new tests.
• No coverage movement beyond the new branches, all asserted.

Reviewer Notes

• Query shape: the correlated scalar subquery (SELECT c.error_type ... ORDER BY c.rowid LIMIT 1) AS family is wrapped in a derived table before the outer GROUP BY — the Fait-vs-spec curve (unwrapped = unknown absorbed) is locked by the oracle test.
• EXPLAIN on the real base: SEARCH c USING INDEX idx_cl_correlation_id, no MATERIALIZE, 40→20 ms, identical rows.
• Overlap: shares read-only proxyRefusalMemory.ts with feat(webhooks): notify when a pool member is set aside #14755 (this PR only reads it); the B6-part symbols + Contingency removal are done in this head.

Maintainer rework (merge-batch 2026-09-24)

• The failure lines reused the traffic sentence (poolEgressObservation) and the empty-state sentence (poolEgressObservationEmpty), so N failures via one exit read "1 exit used by N connections · up to N on one exit" and the unattributed remainder read "No traffic observed in the last 24 h (N)". Added dedicated keys proxyRegistry.poolEgressFailuresByExit / proxyRegistry.poolEgressFailuresUnattributed, translated in all 67 locale files (no __MISSING__ markers).
• tests/unit/ui/PoolEgressObservation.test.tsx now asserts the dedicated keys and that the traffic/empty sentences are not reused (red on the previous head, green after), plus a case where every failure has an exit.
• Merged release/v3.8.51 (clean).