## Linked issue
Closes#5521
## Summary / motivation
Adds an Anki-focused unit testing guide for maintainers, contributors,
and coding assistants.
The guide:
- Defines principles for focused, deterministic, and maintainable tests.
- Covers happy paths, error paths, boundaries, regressions, and
test-first development.
- Provides guidance for Python, Python/Qt, Rust, and
Svelte/TypeScript/JavaScript.
- Clarifies the appropriate scope for UI tests and discourages testing
styling or volatile presentation details.
- Links the guide from the shared `CLAUDE.md`/`AGENTS.md` instructions.