Introduce `nestedUnderJQPath`, enabling subtree extraction using jq-style path parsing. Add associated utility functions (`splitJQPath`, `peelRedundantPathLeaf`, `buildNestedTreeAtPath`) and test coverage. Extend `.env Codec` to support path-specific subtree merges. Update `WithFile` to accept an optional jqPath for targeted merges.
- Relocated testfixtures from the root to the internal package for better encapsulation.
- Updated import paths in codec test files across env, ini, json, yaml directories to reflect the new location.
- Removed the now obsolete testfixtures.go file.