docs(testing): populate the testing pyramid overview (backlog#1153 infra-11) (#4813)

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.

CLAUDE.md

@@ -31,6 +31,8 @@ make build-docker BUILD_OS=ubuntu22.04
 - Architecture, layering, crate map: [ARCHITECTURE.md](ARCHITECTURE.md)
 - Migration guardrails & readiness contracts: [docs/architecture/](docs/architecture/README.md)
 - CI gates: `.github/workflows/ci.yml` (source of truth; never copy its steps into docs)
+- Test-layer taxonomy, per-layer entry commands, serial/nextest rules, flake
+  policy: [docs/testing/README.md](docs/testing/README.md)
 - Tier/ILM transition debugging (xl.meta inspection, versionId tracing):
   [docs/operations/tier-ilm-debugging.md](docs/operations/tier-ilm-debugging.md)