feat(migration): reserve every active client at its current address

kea does not inherit UniFi's lease database. At cutover it starts with an empty
view of who holds what, so it can hand an address that is currently in use to a
different device. Reservations are what carry "this device has this address"
across the switch, because they live in config rather than in lease state.

unifi-reserve-all.py creates one per active client, dry run by default. 51
written, 51/51 verified live by reading the records back; the controller now
holds 85 reservations and the generator emits all 85 with unique, valid
hostnames and no duplicate addresses. Active clients with no reservation went
from 48 to 4.

Three guards, each of which caught something real in the dry run:

  - VRRP virtual addresses are excluded. UniFi reports them as ordinary client
    addresses because the firewalls' bond MACs answer for them, and their
    apparent IP flips between the real interface address and the VIP. Without
    this, 192.168.1.254 -- the gateway VIP itself -- would have been given a
    DHCP reservation.
  - The firewalls' own interface MACs are excluded; those are statically
    configured routers, not DHCP clients.
  - Any address claimed by more than one MAC is dropped rather than guessed
    at. This is how the VIPs surfaced in the first place.

Also skipped: addresses already reserved to another MAC, network gateways, and
anything on a network that does not serve DHCP (which excludes the WAN transit
VLANs automatically).

labsim-dhcp-test.sh gained a lease-database flush, and it is not tidiness. Two
findings, both of which first appeared as a PASSING test:

  - Re-running against stale leases, kea gave dynamic addresses to three
    devices that have reservations. The reservations were present and correct
    in kea's own config throughout. Kea saw the reserved address as leased to
    "another client" -- same MAC, different client-id from the earlier boot --
    and allocated elsewhere. Cutover starts with an empty lease database so
    this is a testing artifact, but a reservation is evidently not
    unconditional once leases exist.
  - Removing only dhcp4-leases.csv does nothing: kea's memfile backend keeps
    lease-file-cleanup rotations (.csv.2) and restores from them on start.

The verdict logic no longer takes the first matching lease row. Doing so
reported an hours-old lease as the current answer and scored three failures as
passes, including one where the device had plainly been given a dynamic
address. A MAC with more than one lease is now an explicit failure rather than
a guess.

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

labsim/README.md

@@ -104,10 +104,32 @@ python3 unifi-to-vyos.py --mode sim -o /tmp/sim.conf   # 6 subnets, 31 mappings
 ```
 
 **Result on VyOS 2026.08 (kea): all four cases pass.** The one that mattered:
-30 of the 31 UniFi reservations sit *inside* the DHCP pool, and **kea honours
-in-pool host reservations** — `printer1` received `172.31.10.46` from within the
+most UniFi reservations sit *inside* the DHCP pool, and **kea honours in-pool
+host reservations** — `printer1` received `172.31.10.46` from within the
 `.10.11–.11.254` pool. That was the open question blocking the cutover.
 
+### The lease database will lie to you
+
+The script wipes `/config/dhcp/dhcp4-leases.csv*` before every run, and both
+halves of that matter:
+
+- **Stale leases defeat reservations.** Re-running against yesterday's leases,
+  kea handed dynamic addresses to three devices that have reservations. The
+  reservation was present and correct in `/run/kea/kea-dhcp4.conf` the whole
+  time. Kea saw the reserved address as already leased to "another client" —
+  same MAC, but a different client-id from the earlier boot — and allocated
+  elsewhere. The cutover itself starts with an empty lease database, so this is
+  a *testing* artifact, but it is worth knowing that a reservation is not an
+  unconditional guarantee once leases exist.
+- **The `*` is load-bearing.** Kea's memfile backend keeps lease-file-cleanup
+  rotations (`dhcp4-leases.csv.2`) and restores from them on start, so
+  truncating only the primary file changes nothing.
+
+Both of those first appeared as a *passing* test. The verdict logic now refuses
+to score a MAC with more than one lease, because taking the first match had
+reported an hours-old lease as the current answer and turned three failures
+into apparent passes.
+
 Still open: whether kea will hand a *reserved* address to a *different* client
 while the reserved device is offline. The negative case here only proves an
 unreserved MAC gets an unreserved address.