docs(migration): record what the rehearsal proved and what it could not

Ran vyos001's real running config plus the real production delta on the labsim
router -- same VyOS version, isolated OVS bridge with no physical NIC, so the
sim briefly holding vyos001's actual addresses could not reach the real LAN.

Result: all 317 commands accepted and the whole delta commits (COMMIT OK), the
revert is byte-exact, and auto-revert fires without rebooting (uptime and
boot-id unchanged across it).

Also written down are the two things this did NOT establish, because a runbook
that overstates its own coverage is worse than one that admits the gap: PPPoE
cannot be tried while the USG holds the single available session, and the
rehearsal ran with vyos001's eth2/eth3 stanzas stripped because the sim VM has
two NICs rather than four.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DMVzWZgiKW2wquf5z8S1yH

migration/CUTOVER.md

@@ -35,6 +35,33 @@ config returns to the previous state and the box does **not** reboot
 
 ---
 
+## What has actually been tested
+
+Proven on the labsim router (same VyOS version, isolated OVS bridge with no
+physical NIC), by loading **vyos001's real running config** and applying the
+**real production delta**:
+
+- All 317 commands accepted, and the whole delta **commits** (`COMMIT OK`).
+- `unifi` mode restores the previous config **byte-exact** (diff clean).
+- Auto-revert fires when the commit is not confirmed: config returns to the
+  saved state and the box does **not** reboot — `uptime` and boot-id unchanged
+  across the revert.
+- Failed health checks trigger an immediate revert rather than waiting out the
+  timer.
+
+Two bugs were found this way and would each have failed the entire switch,
+since the delta commits as one unit: `bond0.51` did not exist for PPPoE to
+reference, and `translation port` rejects a port list.
+
+**Not tested, and untestable in advance:**
+
+- **PPPoE.** The line permits one session and the USG holds it. The first real
+  attempt is during the cutover.
+- **The commit on the real boxes.** The rehearsal ran with vyos001's `eth2` and
+  `eth3` stanzas stripped, because the sim VM has only two NICs. Those are
+  plain interface configs that already work on the real hardware, but they were
+  not part of what committed.
+
 ## Before you unplug anything
 
 1. Tether your workstation to your phone if you want the assistant available.