feat: add L7 graph sync validator

[ChBrain]

L7 - Graph Sync Validator

Implements bidirectional edge consistency checks for the road network graph.

Changes

New validator: scripts/validate_sync.py

• Parses all road indexes (Holds km chain)
• Parses all node Owner blocks (Place: road in state)
• Checks both directions:
  • node claims road R ↔ road R lists this node
  • node claims state S ↔ road R lists node under state S subsection
• Runs independently, re-derives graph from scratch each time

CI Integration: L7-graph-sync job in validate.yml

• Depends on L3, L4, L5 (state, road, node validators)
• Only runs when place files change

Content Fixes:

• A25: regex now accepts [km TBD] placeholders (5 nodes already in indexes)
• A7 elbtunnel:
  • Index entry format fixed: * [km 156](piece_the_kilometerstein.md):...
  • Holds reformatted with standard km marker
  • Distance corrected (0.5 km to Hamburg-Othmarschen)
• A111: tunnel added to index with km TBD
• A40: stretch added to index with km TBD
• A1: two bridges (Volme, Hengstey) added to index with km TBD

Documentation: tests/README.md

• Renumbered layers: L6 = deploy bundles, L7 = graph test, L8 = content samples
• Graph test description updated to reflect implementation

Graph Validation Results

✓ 306 nodes checked, all consistent

Next Steps

• Merge when L7 passes CI
• Consider L8 extension (content samples beyond Withheld section)