Add xenon complexity monitoring and refactor high-complexity code

[cholmes]

Summary

Add code complexity monitoring using xenon and refactor any code that exceeds the target thresholds.

Target Thresholds

• max-absolute: D (no function exceeds D complexity)
• max-modules: C (no module exceeds C average)
• max-average: B (project-wide average stays at B or better)

Long-term goal: Aim for grade 'A' complexity on all new code.

Tasks

1. Add xenon to dev dependencies in pyproject.toml:

   "xenon>=0.9.3",

2. Add xenon pre-commit hook to .pre-commit-config.yaml:

   - repo: https://github.com/rubik/xenon
     rev: v0.9.0
     hooks:
     - id: xenon
       args: ['--max-absolute=D', '--max-modules=C', '--max-average=B']

3. Run xenon to identify high-complexity code:

   xenon --max-absolute=A --max-modules=A --max-average=A src/ftw_dataset_tools/

4. Refactor any functions/modules that exceed the thresholds using:

   • Extract helper functions
   • Early returns (guard clauses)
   • Dictionary dispatch instead of long if-elif chains
   • Single responsibility per function

5. Update CLAUDE.md to remove the TODO marker from the Code Complexity section

Complexity Grades Reference

• A: Simple, easy to understand (TARGET)
• B: Acceptable, low complexity
• C-F: Needs refactoring