feat(trace): add session statistics without event replay

[alectimison-maker]

Summary

• Add a bounded session summary for related trace runs without replaying their event logs in the Traces page.
• Persist content-free per-run counters for model calls, tool activity, retries, errors, token usage, cost, and latency.
• Show the summary alongside the existing conversation turn navigation while preserving the detailed timeline and compare view.

Implementation

• Add browser-neutral pure reducers for event metrics and durable run aggregation, mirrored in Chrome and Firefox.
• Reuse the existing session index for session-scoped run reads and sort results by start time.
• Keep the recorder schema additive: older run records without the new counters remain readable and aggregate as zero for unavailable metrics.
• Keep all counters derived from structured events; no prompt, message, tool argument, or tool result content is added to the persisted summary.
• Do not add another IndexedDB store or change the trace event envelope.

Testing

• node test/run.js — 2003 passed, 0 failed
• npm run test:toolbar-guard — 33 passed
• npm run test:security — 60/60 passed
• node scripts/benchmark-offline-relevance.mjs — completed successfully
• Node syntax checks and git diff --check — passed
• Manual unpacked-extension verification was not run in this environment

Compatibility

• Existing detailed event timelines, exports, compare mode, automatic refresh, repair flow, and privacy defaults remain unchanged.
• Chrome and Firefox use the same statistics reducer and retain their existing platform-specific recorder behavior.
• Session reads remain bounded by the current viewer limit and fall back to the existing chronological scan when the session index is unavailable.

Related changes: #2877, #2889, #2893.

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