add: spec 166 - fix LCOV function name matching for Rust methods

Addresses false positive [ERROR UNTESTED] flags caused by function name
mismatch between AST extraction and LCOV demangling.

Root cause: Debtmap extracts simple method names (e.g., 'create_auto_commit')
but LCOV contains fully-qualified paths after demangling (e.g.,
'prodigy::cook::CommitTracker::create_auto_commit').

Solution:
- Enhanced normalization to extract both full path and method name
- Multi-strategy matching (exact, line, method name, suffix)
- Debug tool: explain-coverage CLI command
- Integration tests with real Prodigy LCOV data

Target: Reduce false positive rate from >50% to <10%

🤖 Generated with Claude Code

Author: iepathos