What

Raise logger test coverage from ~65% (README claim) to 80%, closing gaps in rotating writer, async queue drain, and structured-field formatters.

• Current: ~65% line coverage
• Expected: >=80% line, >=70% branch
• Scope: tests/**

Why

• Async drain paths are the most frequent source of field-report bugs
• Structured logging (v3.1.0+) added paths without matching tests
• Brings logger parity with common_system (80%+) and network_system (~80%)

How

Technical Approach

1. gcovr diff against last release; list top 15 gaps
2. Add GTest cases + one property-based test for async drain under shutdown
3. Enforce 80% gate in .github/workflows/coverage.yml

Acceptance Criteria

• Coverage >= 80% line / 70% branch
• CI gate enforced
• README badge reflects measured value