test, expand coverage from ~77% to above 94%

[steven-varga]

Current line coverage is 76.2% (250/328). The biggest gaps are:

1. backend: and branch are completely untested (0%, ~50 lines).
2. mode: stale-file detection pass/fail paths are untested (~15 lines).
3. Edge cases: duplicate struct dedup and matcher branch (~4 lines).

Add fixtures and CTest invocations to cover these paths. Target: >94% line coverage.