fix(topology/allocator): widen default subnet base to /12 for mass-scale

A 30-LAN generate request already fits in 172.20.0.0/16, but trees
with depth/branching that multiply past 256 (e.g. depth=6,
branching=4 ≈ 5k LANs) hit AllocatorExhausted before the first
write.

SubnetAllocator now accepts a full CIDR base ("172.16.0.0/12" →
4096 /24s) in addition to the legacy two-octet shorthand ("172.20",
auto-lifted to /16). The parent must be ≤/24; a /24 base yields
exactly one slot. Iteration order is preserved for /16 bases so
existing topologies keep their third-octet sweep; /12 adds a
second-octet dimension underneath.

Defaults bumped to 172.16.0.0/12: TopologyConfig.subnet_base_prefix,
/next-subnet query param, and the mutator's add-LAN fallback. The
field pattern widens to accept CIDR. create-blank and manual LAN
CRUD still use "10.0" (lifts to /16) — one DMZ LAN per topology,
256 is plenty.

decnet/mutator/ops.py

@@ -118,7 +118,7 @@ async def apply_add_lan(
 
     if subnet is None:
         reserved = await reserved_subnets(repo)
-        alloc = SubnetAllocator(base_prefix="172.20", reserved=reserved)
+        alloc = SubnetAllocator(base_prefix="172.16.0.0/12", reserved=reserved)
         subnet = alloc.next_free()
 
     await repo.add_lan(