## Linked issue
Refs #4838#4839#4840
## Summary / motivation
Adds a contributor-facing guide for the test coverage setup introduced
across the three coverage PRs, and updates CLAUDE.md to reflect that
`just` is now the single entry point for all build, test, lint, and
format commands.
## How to test
- Read `docs/testing-coverage.md` — verify it covers all three stacks,
thresholds, and the known gaps section.
- Run `just docs` and open `out/docs/html/index.html` to confirm
`testing-coverage` appears in the sidebar under Contributing.
- Read `CLAUDE.md` — verify `./ninja` / `./tools` references are gone
and `just test-rust`, `just test-py`, `just test-ts` are mentioned.
### Details
- `docs/testing-coverage.md`: documents tools, thresholds, per-stack
entry points, and known gaps for all three coverage stacks.
- `docs/index.md`: wired `testing-coverage` into the Sphinx toctree
after `contributing`.
- `CLAUDE.md`: replaced raw `./ninja` / `./tools` invocations with
`just` equivalents; added top-level note directing contributors to
`just --list`; added `just test-rust`, `just test-py`, `just test-ts`
to the Quick iteration section.
---------
Co-authored-by: Abdo <abdo@abdnh.net>
