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/topology/config.py

@@ -30,8 +30,16 @@ class TopologyConfig(BaseModel):
     # from its LAN to a non-parent, non-child LAN.  0.0 yields a tree.
     cross_edge_probability: float = Field(default=0.0, ge=0.0, le=1.0)
 
-    # IP allocation base.  LANs get sequential /24s starting here.
-    subnet_base_prefix: str = Field(default="172.20", pattern=r"^\d{1,3}\.\d{1,3}$")
+    # IP allocation base.  LANs get sequential /24s carved out of this
+    # network.  Accepts either a full CIDR (e.g. ``172.16.0.0/12`` for
+    # 4096 slots) or the legacy two-octet shorthand ``172.20`` which
+    # auto-lifts to ``172.20.0.0/16`` (256 slots).  Default is a /12
+    # so mass-scale topologies (depth/branching trees with >256 LANs)
+    # don't exhaust the pool on first generation.
+    subnet_base_prefix: str = Field(
+        default="172.16.0.0/12",
+        pattern=r"^\d{1,3}\.\d{1,3}(\.\d{1,3}\.\d{1,3}/\d{1,2})?$",
+    )
 
     # Service selection — reuses decnet.fleet.build_deckies' randomizer.
     randomize_services: bool = Field(default=True)