refactor(#2363): single source of truth for DHCP pool size

Move the DHCP pool size constant out of the two separate definitions
(Nix literal + Rust const) into a shared data file: nix/dhcp-pool-size.

- nix/dhcp-pool-size: new file, contains '14'
- hive-gateway.nix: reads via builtins.readFile + toIntBase10
- lifecycle/mod.rs: parses via include_bytes! const block at compile time

Cargo automatically tracks include_bytes! as a file dependency so a
change to nix/dhcp-pool-size triggers recompilation without build.rs.

nix/modules/hive-gateway.nix

@@ -15,9 +15,11 @@ let
   # Occupies the last dhcpPoolSize usable addresses of the subnet
   # (e.g. .241-.254 on a /24). Agent containers use deterministic static
   # IPs (lifecycle::agent_network_ip) and are excluded from this range
-  # by a remap in the Rust code. Must stay in sync with DHCP_POOL_SIZE
-  # in hive-c0re/src/lifecycle/mod.rs.
-  dhcpPoolSize = 14;
+  # by a remap in the Rust code.
+  #
+  # Single source of truth: `nix/dhcp-pool-size` (one integer).
+  # Rust reads it at compile time via `include_bytes!` in lifecycle/mod.rs.
+  dhcpPoolSize = lib.strings.toIntBase10 (lib.strings.trim (builtins.readFile ../dhcp-pool-size));
   # IPv4 helpers — nix integers are 64-bit so all /0-/32 values are safe.
   ipToInt =
     ip: