Build test_count_cache once before ranking instead of calling
existing_unit_test_count O(2N) times. Guard for None function_to_tests
and add debug logging when effort is escalated from medium to high.
- Add existing_unit_test_count() with parametrized test deduplication
- Stable-sort ranked functions so tested ones come first
- Enable reference graph resolver (was disabled) for non-CI runs
- Add per-function logging with ref count and test count
- Auto-upgrade top N functions to high effort when user hasn't set --effort
- Add CallGraph model with traversal (BFS, topological sort, subgraph)
- Add get_call_graph() to DependencyResolver protocol and ReferenceGraph
- Refactor get_callees() to delegate through get_call_graph()
CF-1660