chore(m169): close — spec done, rubric graded, architecture carries the write floor

All 16 Dimensions DONE; rubric fully graded (R1–R6, S1–S7). The sandbox
boundary section of runner_fleet.md gains the write-floor derivation and the
TempFileCreateFailed record, mirroring how the read side documented its own
incident.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>