feat(splitter): targeted caller detection, sub-split, scored support files

- Replace loose diff.contains(sym_name) merge with targeted caller
  detection: only merge when a +line contains sym_name( pattern
- Post-clustering sub-split: groups >6 files spanning multiple modules
  are automatically split by module to prevent mega-groups
- Scored support file assignment with known pairs (Cargo.toml+Lock,
  package.json+lock, etc.), stem overlap scoring, and standalone
  fallback for weak-affinity files
- Add diff-shape fingerprinting with centroid-based clustering
- Expand GENERIC_DIRS to 22 common directory names

Author: Sephyi