docs(testing): populate testing pyramid, naming, serial/nextest rules
Fill the docs/testing/README.md skeleton (backlog#1153 infra-11):
- Test taxonomy table for all eight layers (unit / ecstore black-box /
e2e / s3s-e2e / S3 compatibility / chaos / fuzz / bench) with a
verified entry command and a qualitative "when it runs" per layer; the
event x timeout x required-status matrix stays owned by
docs/testing/ci-gates.md (ci-15), linked not duplicated.
- Naming conventions section with the migration-gate reserved substrings
(data_movement / rebalance / decommission / source_cleanup /
delete_marker) linking the infra-12 count-floor guard, closing that
task's docs cross-link.
- Serial execution & nextest rules: nextest as a hard dependency with the
RUSTFS_ALLOW_CARGO_TEST_FALLBACK escape hatch and the runner-semantics
difference (folds the infra-14 README half), why #[serial] is a no-op
under nextest, and the default/ci/e2e-smoke/e2e-repl-nightly profiles.
- A time-control placeholder for infra-4 to fill.
The pre-existing flake-policy section (ci-10) is preserved verbatim.
Add pointers from CLAUDE.md and CONTRIBUTING.md.
