What

Increase logger_system test coverage from 65% to 70%+ minimum threshold required for v1.0 release.

Part of #604

Why

• Current coverage: 65% (below 70% minimum for v1.0)
• Test suite has 47 unit + 5 integration + 10 benchmark tests
• Need to identify and cover untested code paths to ensure release quality

How

Technical Approach

1. Identify coverage gaps using CI coverage reports
2. Prioritize testing for:
   • Core logger operations (src/core/)
   • Writer implementations with low coverage (src/impl/writers/)
   • Error handling paths
   • Edge cases in public API
3. Add targeted unit tests for uncovered code paths
4. Verify coverage improvement via CI

Acceptance Criteria

• Coverage reaches 70%+ as reported by CI
• New tests follow existing test patterns and conventions
• All existing tests continue to pass
• No test flakiness introduced