fix(session): order messages across ID rollover

[HudsonGri]

Summary

• order session messages by persisted creation time, with ID only as a same-millisecond tie-breaker
• use the same comparator for loop completion and pending compaction/subtask selection
• add a rollover fixture with IDs immediately before and after the 48-bit timestamp boundary
• prove SQLite pagination already preserves chronological storage order across the boundary

Why

Ascending IDs encode Date.now() * 0x1000 + counter into six bytes. The timestamp prefix wraps every 2^36 ms. At the next boundary, 2026-08-14T11:19:55.136Z, a newer ID sorts lexically before an older ID. Long-lived sessions can therefore select the wrong latest user/assistant and fail the loop-exit check.

Verification

• bun test test/session/message-v2.test.ts test/session/messages-pagination.test.ts (89 pass)
• bun run typecheck

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The following comment was made by an LLM, it may be inaccurate:

Based on my search results, no duplicate PRs were found that are addressing the same issue as PR #40432. While there are some related PRs about session message ordering (like #38798 "order messages by time so the run loop can terminate" and #40099 "finish prompt loop by parent link"), none appear to be duplicates of this specific PR which focuses on fixing the ID rollover issue with 48-bit timestamp boundaries.

No duplicate PRs found

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