feat(dhcp): add DhcpServerMixin for static DHCP reservations

Adds the generic half of DHCP reservation management: diff_dhcp_reservations
matches desired against live reservations by normalised MAC, and
apply_dhcp_reservationset walks the diff and commits once at the end.

Both are concrete here because neither is vendor-specific — only
get_dhcp_reservations/apply_dhcp_reservation/commit_dhcp_reservations touch
the device (Kea REST on OPNsense, dnsmasq/odhcpd UCI on OpenWrt).

Two deliberate choices:

- The MAC is the matching key, not a description as with firewall rules. A
  reservation has a natural identity and this is it. That also means a host
  moving to another VLAN is an update of the existing entry rather than a
  second one for the same MAC.
- An empty diff skips the commit. Committing reloads the DHCP daemon and
  drops in-flight requests, which is too high a price for a no-op run. This
  differs from apply_firewall_ruleset, which always commits.

Live reservations with no desired counterpart are never reported for
deletion — a DHCP server routinely carries hand-created entries the caller's
desired set was never meant to describe.

Mixed into FirewallDriver and ResidentialGatewayDriver: both device types
commonly run the DHCP server for their networks.

README.md

@@ -56,6 +56,12 @@ class FirewallDriver(DeviceTypeDriver):
         ...  # computes the diff, calls apply_firewall_rule() per change, commits
 ```
 
+The same split applies to `DhcpServerMixin`: `get_dhcp_reservations`/
+`apply_dhcp_reservation`/`commit_dhcp_reservations` are abstract (Kea REST on
+OPNsense, dnsmasq/odhcpd UCI on OpenWrt), while `diff_dhcp_reservations` and
+`apply_dhcp_reservationset` are concrete — matching by normalised MAC and the
+apply-then-commit orchestration are identical for every DHCP server.
+
 A new driver (FortiGate, pfSense, …) gets `diff_firewall_rules`/
 `apply_firewall_ruleset` for free the moment it implements the three abstract
 methods — it never needs to reimplement the reconciliation logic itself.
@@ -103,6 +109,13 @@ Requires Python ≥ 3.9 and NAPALM ≥ 4.0.
 | `HypervisorDriver` | Hypervisors & virtualisation platforms | Proxmox VE, VMware ESXi, KVM/libvirt |
 | `OSDriver` | General-purpose operating systems | Linux, BSD, macOS |
 | `StorageDriver` | Storage appliances & NAS/SAN | TrueNAS, Synology DSM, QNAP QTS |
+| `ResidentialGatewayDriver` | Router + firewall + AP in one box | OpenWrt, FritzBox |
+
+Mixins mixed into the classes above rather than used on their own:
+`ConfigLifecycleMixin` (config load/compare/commit/rollback), `PingSweepMixin`
+(subnet sweeps), and `DhcpServerMixin` (static DHCP reservations — mixed into
+`FirewallDriver` and `ResidentialGatewayDriver`, since both commonly run the
+DHCP server for their networks).
 
 ## Usage