Upgrade EdgeRange iterator to bidirectional iterator

[Copilot]

Upgrades the EdgeRange iterator from forward-only to bidirectional, enabling backward iteration through graph edges.

Changes

• Iterator category: Upgraded from std::forward_iterator_tag to std::bidirectional_iterator_tag
• Backward iteration operators: Implemented operator--() (prefix) and operator--(int) (postfix)
• CSR backward traversal: Added retreat_to_prev_valid_csr() helper that:
  • Decrements within vertex adjacency lists
  • Navigates to previous vertices when at adjacency list boundaries
  • Handles decrement from end() iterator
  • Skips empty vertices during backward traversal
• Tests: Added bidirectional iteration tests in parameterized format for both EdgeList and CSR representations

Test Coverage

42 test cases (7 generators × 6 test variations):

• 4 forward iteration tests per generator
• 2 bidirectional iteration tests per generator

All tests pass ✅

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.