docs: Add comprehensive test coverage strategy document

Create TMP_TESTCOV.md with detailed plan to achieve ~100% meaningful
test coverage including:

- Current state analysis (CLI ~70%, Reporter ~60%, Flask ~40%)
- Priority analysis of 15+ untested Python methods
- Negative path, edge case, and corner case testing strategies
- CI optimization techniques (parallelization, caching, splitting)
- 12-step implementation plan across 5 phases
- Test templates and conventions for Python → TypeScript migration
- Shared test data approach for migration parity

Special focus on Python reporter module in preparation for TS migration.

Author: claude