Add comprehensive ERT test suite and fix critical bugs

Session 1: Testing infrastructure and initial test coverage

Bug fixes in wttrin.el:
- Fix wttrin-additional-url-params to handle nil unit system
- Remove incorrect callback parameter to url-retrieve-synchronously
- Add nil buffer check for network failures
- Strip HTTP headers before decoding response
- Kill buffer after fetch to prevent memory leaks
- Fix double concatenation of URL params in cache function
- Add proper URL encoding via new wttrin--build-url function

Refactoring:
- Extract wttrin--build-url as pure, testable function
- Separate URL building logic from network I/O

Test infrastructure (33 tests, 100% passing):
- tests/testutil-wttrin.el: Shared test utilities
- tests/test-wttrin-additional-url-params.el: 7 tests
- tests/test-wttrin--make-cache-key.el: 9 tests
- tests/test-wttrin--build-url.el: 10 tests
- tests/test-wttrin--cleanup-cache-if-needed.el: 7 tests

Documentation:
- docs/testing-plan.org: Comprehensive testing roadmap
- docs/bugs.org: Bug analysis from code review
- docs/NOTES.org: Session tracking and guidelines
- docs/session-1-summary.org: Detailed session summary

Next session: Cache workflow tests, parsing logic extraction, integration tests

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

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