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.
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