docs: Add comprehensive test-reporter specification

Created detailed specification for test reporting system that will:
- Parse ERT batch output and generate test summaries
- Show total tests, passed, failed, duration
- List individual test failures with details
- Enable incremental improvement via TDD

Key Design Decisions:
- Language: Emacs Lisp (dogfooding, no dependencies, better integration)
- Architecture: Collect → Parse → Report
- Integration: Makefile + test-runner.el
- Test isolation: Clear tests on project switch

Implementation Phases:
- Phase 1: Basic stats (tests, passed, failed, duration)
- Phase 2: Failure details (messages, rerun commands)
- Phase 3: Rich details (timing, slowest tests, assertions)

Cross-Project Test Isolation:
- Problem: ERT tests globally registered across all projects
- Solution: Clear tests on project switch + project-aware commands
- Hybrid approach combining automatic and manual control

Integration Points:
- Makefile: Pipe output to test-reporter script
- test-runner.el: Post-run hooks or result wrapping
- ERT: Custom reporter (future enhancement)

Test-Driven Development:
- Parser tested with real ERT output samples
- Fixtures for edge cases
- Incremental improvement as new formats encountered

This is marked as NEXT PRIORITY TASK.

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