(index): continuation scan loops forever when the stored and stat mtimes round to different milliseconds #891

Found in the review of #890 (pre-existing on main), CONFIRMED by a local probe (Node 22, Windows).

What happens. scanContinuationFile (session-cache.js ~98-110) can loop forever on a transcript larger than one 1 MiB continuation chunk.

Why. Two rounding rules disagree on the millisecond:

• scanCachedContinuation (session-cache.js ~180-181) stores index.mtime by rounding the BigInt mtimeNs.
• scanContinuationIndex (session-continuations.js ~36-37) reuses the saved cursor only if index.mtime === stat.mtime.toISOString(). stat.mtime comes from the floating-point mtimeMs, whose precision step is about 244 ns at current dates.

When the sub-millisecond part sits just under .5 ms, the two land on different milliseconds. The cursor is then never reused. Each pass restarts from offset 0 with the same 1 MiB budget, bytes < size stays true, and the do…while never exits; it re-reads 1 MiB per setImmediate until the file changes.

Probe. Same fixture as the D2 test, mtime set to an exact tick with SetLastWriteTimeUtc:

• mtimeNs …499900 (bigint rounding .000Z, stat.mtime .001Z): prepareContinuationRefresh still not done after 4 s, after 1926 opens of the file.
• …499800: done after 8 opens.

Impact.

• A finished session's transcript never changes, so the loop never ends.
• In the writer worker (ADR 0004), cacheTail queues every later computation behind it, so the index stops updating.
• Roughly 1 in 10⁴ multi-chunk files hit it, at random.
• It is the plausible cause of the 9 s D2 timeout on CI ((index): tighten the #733 continuation test and cover the post-header stat check (follow-up #878) #879, run on (schedules): narrow the sandbox and PATH hints and keep the manual-run label (#877) #883), not proven.

Fix. Compare stamps (mtimeNs, or the same BigInt-derived value) instead of ISO dates, or derive both values with the same rounding. Red first: a test with an mtime on the ns boundary that hangs before the fix and finishes after.