refactor(timeline): parse each usage timestamp once before sorting

[seonghobae]

Current exact authority

• protected base: developmental@2fa92012bcf80acc1f921a4bafea76b3b1424b46
• exact head: e1dd7cc5c6ce9cc33990b4f0fc5b4e80dfae7e5e
• lifecycle: Draft / structural parse-count contract added / hosted gates non-terminal
• effective delta: session-timeline-chart.tsx plus its focused test only

Finding and causal repair

The generated branch correctly recognized repeated Date.parse() inside the sort comparator, but its implementation discarded the pre-parsed key immediately after sorting and parsed every usage timestamp again during the merge pass. It therefore reduced comparator-time parsing but did not realize the intended one-parse-per-usage boundary.

The current descendant keeps { usage, parsedTimestamp } through sorting and reuses that primitive as the merge cursor timestamp. A focused regression spies on Date.parse with three unsorted usage rows and no tool events, requires exactly one parse per usage row, and verifies chronological output. Existing cumulative tool-summary and input-order immutability tests remain.

Evidence discipline

The branch-generated .jules/bolt.md rule and source comment generalized this into an O(N) sorting optimization and claimed hot-loop performance without a committed representative benchmark. That is not correct: sorting remains O(N log N); only timestamp parsing for usage sort keys is bounded to one O(N) pass. The repository-wide Bolt doctrine was restored byte-for-byte to protected authority and the unmeasured source performance claim was removed.

This PR therefore claims only the deterministic operation-count improvement and behavior preservation. Buyer-path latency, allocation/GC reduction, or material render-speed improvement require separate representative benchmark/profile evidence before being asserted.

Exact-head gates

Fresh workflows for e1dd7cc5... are non-terminal: CI 33997964975, Security Scan 33997965035, SAST Semgrep 33997964993, CodeQL PR 33997964996. Predecessor checks are not transferred. Keep Draft until the unchanged head has terminal applicable gates, zero valid unresolved findings, fresh protected-base compatibility, and the then-live independent-review requirement.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch bolt/optimize-timeline-sort-10957893818914055786

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