measure-lab: preserve session warning history and clarify prior-session export #177

Nonblocking Greptile findings from PR #7 at c959c13, deferred under the 08:30Z standard.

• 4115045459: SessionSheet exports only the current session, but PROTOCOL.md defers sharing until after several sessions. Add prior-session export or correct the sequence. Earlier folders remain available through Files, which the sheet documents; no data loss was established.
• 4115045463: LabSession.openFolder clears a previously displayed keyframe-write warning. Preserve per-session failure history. Failed live-tap and freeze writes stop before accepting the operation, and failed motion writes release the spacing reservation for retry. Clearing the old warning on an explicit new session does not itself delete saved evidence or create a false accepted measurement.

The misleading validation pass display is a separate open blocker, not part of this follow-up.