docs: Document test expansion removal and lessons learned

Added TEST_EXPANSION_STATUS.md explaining why test expansions were removed:
- 48 test failures due to parameter conflicts
- Missing golden reference files
- Riemann solver constraints (solver 3, 4)
- Non-zero exit codes don't help coverage

Removed outdated documentation:
- TEST_SUITE_EXPANSION_IMPLEMENTED.md
- CI_COVERAGE_IMPLEMENTATION_COMPLETE.md

Branch remains valuable for:
- CI configuration (80% threshold)
- Coverage tooling and scripts
- Comprehensive documentation
- -a flag usage (+21.6% coverage)

Test count: Back to 459 (stable baseline)
All tests passing, ready for production.

Author: sbryngelson