Add configuration file splitting rule to §2
Summary
Adds §2.x "Configuration File Structure" to the policy, formalising when nix/configuration.nix should be split into multiple files under nix/configuration/.
Based on the sheaf-theoretic analysis from @fancypants and formalisation by @nastypants in #1:
- Split when definitions have disjoint support (non-overlapping option paths) AND non-trivial internal structure
- Validity test: each file must be self-contained without cross-references
- Counter-indication: don't split trivially small definitions
- Partition modules belong in
nix/partitions/, not alongside configuration