New docs/src/development/testing.md covering:
- Quick reference commands (pixi run test-kindred, pixi run test)
- Tier 1 pure Python tests: mock strategy, test file table, addon
loader test class breakdown, how to write a new test file
- Tier 2 FreeCAD headless tests: runner, current status
- C++ GoogleTest infrastructure: directory layout, test executables
- Decision guide for which tier to use
Add entry to docs/src/SUMMARY.md under Development section.
