test/snapshot-mirror.test.ts's off-box mirror cases pass every time on a loaded check machine #4580

"the off-box mirror > …" in test/snapshot-mirror.test.ts went red on pulls #3821 and #3959 — pulls that never touched its code. It depends on wall-clock time or ordering on a loaded check machine.

Done:

• [ ] both "a snapshot lands in both buckets" and "an unreachable mirror is one line on the board" wait on the upload's completion
• [ ] it waits on the event it means (a settled promise, a row, a ref), never a fixed sleep or a raw deadline; any time budget goes through the load-proof helper from #4431
• [ ] bun test test/snapshot-mirror.test.ts passes 20 times in a row under scripts/flake-hunt.ts (or with 8 copies running at once), and the pull says how it was run