feat(trace): add step trajectory table

[alectimison-maker]

Summary

• Add a compact step summary to the Traces page above the existing detailed event timeline.
• Group durable trace events by actual step and run lifecycle, showing status, duration, token usage, cost, tool activity, vision sub-calls, error codes, and repair markers.
• Keep Chrome and Firefox behavior synchronized through a browser-neutral aggregation module.

Related changes: #2877, #2889.

Implementation

• Add a pure buildTraceTrajectory(events) function for deterministic aggregation without changing the recorder schema or adding another IndexedDB projection.
• Represent turn lifecycle events in a run-level row so the lifecycle marker at step zero does not appear as a permanently running model step.
• Preserve older traces without lifecycle boundaries with an unknown status instead of guessing completion from response payloads.
• Keep the existing detailed timeline, compare mode, export behavior, lossless warning, and interrupted-run repair flow unchanged.
• Reuse existing localized labels and display only bounded aggregate/error evidence; raw request and tool payloads are not added to the new table.

Testing

• node test/run.js — 1999 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 Chrome unpacked-extension smoke test — Traces opened, Refresh worked, the step summary rendered before the timeline, and there were no console or page errors
• Firefox covered by mirrored source, parity tests, syntax checks, and the shared suite; manual Firefox verification was not run

Compatibility

• The change is additive and reads the existing event log without changing stored event shapes.
• Existing trace exports and privacy warnings remain unchanged.
• The table remains bounded by the current trace viewer's normal run/event limits.

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