Previously `pnpm run test` always ran the full frontend-tests suite,
which made tight iteration on a single namespace or var painful. The
runner now accepts `--focus <ns>` or `--focus <ns>/<var>` and executes
only the matching tests, preserving each namespace's `:once` and `:each`
fixtures so behavior matches a full-suite run.
pnpm run test -- --focus frontend-tests.logic.groups-test
pnpm run test -- --focus frontend-tests.logic.groups-test/some-test
Also updates the developer guide and the testing memory so the flag is
discoverable from both docs and agent context.