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.

docs/testing/README.md

@@ -1,15 +1,67 @@
 # RustFS Testing
 
 > **Owner: backlog#1153 (infra-11).** This file is the authoritative home for
-> test taxonomy, naming conventions, and serial/quarantine rules. It is
-> currently a **skeleton** — infra-11 fills in the remaining sections. The
-> event × budget × required matrix lives in `docs/testing/ci-gates.md`
-> (ci-15); this file links to it rather than duplicating counts.
+> the test-layer taxonomy, naming conventions, and serial/nextest rules. The
+> event × budget × required-status matrix is owned separately by
+> `docs/testing/ci-gates.md` (backlog#1149 ci-15); this file links to it rather
+> than duplicating counts, timeouts, or required-check names.
 
 ## Test taxonomy
 
-_TODO (infra-11): unit / e2e-smoke / e2e-full / e2e-nightly / protocols /
-s3-tests / fuzz / perf, with naming conventions._
+RustFS layers its tests from cheap-and-narrow to expensive-and-broad. Higher
+layers catch what lower layers cannot but cost more wall-clock and setup, so
+each layer has a clear entry point and a clear "when". Pick the lowest layer
+that can prove your change; add a higher-layer test only when the behaviour is
+not observable below it.
+
+| Layer | What it covers | Entry command | When it runs |
+|---|---|---|---|
+| Unit & crate integration | Per-crate logic and in-process integration tests, run under nextest | `cargo nextest run --all --exclude e2e_test` (or `-p <crate>`) | Every PR (required) |
+| ecstore black-box | Erasure-coded read/write/recovery validation of the ecstore stack | `scripts/run_ecstore_validation_suite.sh --profile quick` | Local / release validation (not in CI workflows) |
+| e2e (`e2e_test` crate) | Full server spun up per test, driven over the S3 API | `cargo nextest run --profile e2e-smoke -p e2e_test` | PR smoke lane + scheduled full/nightly lanes |
+| s3s-e2e conformance | External S3 conformance tool run against a live rustfs server | `./scripts/e2e-run.sh ./target/debug/rustfs /tmp/rustfs-e2e-data` | Per-PR e2e gate (`e2e-tests` job) |
+| S3 compatibility | Third-party suites: `ceph/s3-tests` (boto3) and MinIO `mint` (many SDKs) | `scripts/s3-tests/run.sh` (mint: `.github/workflows/mint.yml`) | s3-tests: per-PR gate; mint: scheduled, report-only |
+| Chaos / fault-injection | Multi-node, power-loss, and disk-fault harness | — (harness planned) | Planned — tracked in backlog#1100 |
+| Fuzz | `cargo-fuzz` targets over untrusted parsing/validation surfaces | `./scripts/fuzz/run.sh` (or `cd fuzz && cargo +nightly fuzz run <target>`) | PR smoke + nightly corpus (`.github/workflows/fuzz.yml`) |
+| Benchmarks | Criterion micro/throughput benchmarks | `cargo bench -p <crate>` | On-demand / local |
+
+> The **When it runs** column is a qualitative pointer only. The authoritative
+> event × timeout × required-status matrix lives in `docs/testing/ci-gates.md`
+> (ci-15) — do not duplicate its numbers here.
+
+Layer notes:
+
+- **Unit & crate integration** — the primary gate. `make test` wraps
+  `cargo nextest run --all --exclude e2e_test`; CI runs the same set under the
+  strict `ci` profile (`.config/nextest.toml`). Requires `cargo-nextest` (see
+  [Serial execution & nextest profiles](#serial-execution--nextest-profiles)).
+- **ecstore black-box** — `scripts/run_ecstore_validation_suite.sh` has four
+  profiles (`quick` / `full` / `destructive` / `fuzz`); `quick` is the
+  PR-smoke-sized core read/write/recovery pass. It is a local and
+  release-validation tool, not wired into a CI workflow.
+- **e2e** — each test spawns its own single-node rustfs server on a random port
+  with an isolated temp dir, so the suite is parallel-safe. The `e2e-smoke`
+  profile is the single PR wiring mechanism; slow/cross-process suites run in
+  scheduled lanes (`e2e-repl-nightly`, and the reliability group). Full
+  contributor guide: [`crates/e2e_test/README.md`](../../crates/e2e_test/README.md);
+  per-module counts: [`e2e-suite-inventory.md`](e2e-suite-inventory.md).
+- **s3s-e2e** — an external black-box conformance tool installed in CI; locally
+  run it against a freshly built binary with `scripts/e2e-run.sh <binary>
+  <data-dir>`.
+- **S3 compatibility** — `ceph/s3-tests` exercises S3 semantics through boto3
+  and is gated on a committed allow-list
+  (`scripts/s3-tests/implemented_tests.txt`); MinIO `mint` runs many real
+  client SDKs and is report-only until suites pass reliably (see the header of
+  `.github/workflows/mint.yml`).
+- **Chaos / fault-injection** — multi-node, power-loss, and disk-fault
+  scenarios are out of scope for this repo's in-tree suites; the harness is
+  tracked in backlog#1100. (Single-node disk-fault e2e tests already live in the
+  e2e crate's `e2e-reliability` group.)
+- **Fuzz** — the `cargo-fuzz` harness is an isolated sub-workspace under
+  `fuzz/` (kept out of the root workspace on purpose); see
+  [`fuzz/README.md`](../../fuzz/README.md) for targets and corpus rules.
+- **Benchmarks** — Criterion benches live under each crate's `benches/`. They
+  are not a gate; run them locally to compare before/after on a specific crate.
 
 ### Security advisory regression tests
 
@@ -18,10 +70,87 @@ advisory -> test map lives in
 [`docs/testing/security-regressions.md`](security-regressions.md). sec-14
 (backlog#1151) formalizes the written admission policy in `AGENTS.md`.
 
-## Serial groups & CI profiles
+## Naming conventions
 
-_TODO (infra-11): document the `[test-groups]` mechanism and the `default` vs
-`ci` nextest profiles. See `.config/nextest.toml`._
+### Reserved test-name substrings (migration gate)
+
+The migration-critical CI gate selects tests **by name substring** rather than
+by module path, so a rename that drops the substring silently thins the gate.
+These substrings are therefore reserved: keep them in the test name when a test
+proves migration-critical behaviour.
+
+| Substring | Guards |
+|---|---|
+| `data_movement` | Cross-pool / cross-set object data-movement proofs |
+| `rebalance` | Pool rebalance correctness |
+| `decommission` | Pool decommission correctness |
+| `source_cleanup` | Post-migration source cleanup |
+| `delete_marker` | Delete-marker handling across migration |
+
+The gate is enforced by
+[`scripts/check_migration_gate_count.sh`](../../scripts/check_migration_gate_count.sh),
+which counts the selected tests and fails if the count drops below the committed
+floor in
+[`.config/migration-gate-floor.txt`](../../.config/migration-gate-floor.txt).
+A deliberate reduction must lower the floor in the same PR, so the change is
+reviewable in the diff (backlog#1153 infra-12). The substring list above is the
+same one the gate uses; keep the two in sync when adding a reserved word.
+
+### General naming
+
+- Name a regression test after what it pins: the issue or advisory number
+  (`..._regression_test`, `..._issue_NNNN_...`) or the invariant it protects, so
+  a reviewer can find the guard for a past bug by grepping.
+- e2e lane membership is driven by test-name patterns in `.config/nextest.toml`
+  (for example the `_real_dual_node` / `_real_single_node` markers route
+  replication tests into the nightly lane). Follow the existing marker when
+  adding a test to an established suite; see
+  [`crates/e2e_test/README.md`](../../crates/e2e_test/README.md).
+- Follow the Rust API Guidelines for symbol naming (see `AGENTS.md`).
+
+## Serial execution & nextest profiles
+
+**`cargo-nextest` is the runner.** `make test` requires it and CI installs it;
+plain `cargo test` is not a faithful substitute because the two runners execute
+tests differently.
+
+- **Install:** `cargo install cargo-nextest --locked`, or a prebuilt binary
+  (faster) from <https://nexte.st/docs/installation/>.
+- **Escape hatch:** `RUSTFS_ALLOW_CARGO_TEST_FALLBACK=1 make test` runs the
+  plain `cargo test` fallback, but the results are **not authoritative** —
+  serialization semantics differ from CI and `[test-groups]` do not apply.
+
+**Why `#[serial]` is mostly a no-op under nextest.** nextest runs every test in
+its own process, so `serial_test`'s in-process `#[serial]` mutex does **not**
+serialize tests against each other. The mechanism that actually serializes
+across nextest's process boundary is a nextest `[test-groups]` entry with
+`max-threads = 1` (for example `ecstore-serial-flaky` and `e2e-reliability` in
+`.config/nextest.toml`). Consequences:
+
+- Do not add `#[serial]` expecting cross-test isolation under nextest. If two
+  tests genuinely share process/global state or a fixed external resource,
+  serialize them with a `[test-groups]` entry, or make each test self-isolating
+  (per-test instance context — see backlog#1153 infra-7 / infra-8).
+- A large share of the repo's existing `#[serial]` markers only affect the
+  `cargo test` fallback. The serial-debt census and removal plan are tracked in
+  backlog#1153 infra-7.
+
+**Profiles** (all defined in `.config/nextest.toml`):
+
+- `default` — local runs. **Never retries**: a red test locally is a real
+  failure to investigate, not noise to retry away.
+- `ci` — the strict CI gate: global `retries = 0` plus a narrowly-scoped
+  quarantine list (`retries = 2`) for tests with a tracked OPEN flake issue.
+- `e2e-smoke` — the PR smoke subset of the `e2e_test` crate (the single wiring
+  mechanism for e2e in PR CI).
+- `e2e-repl-nightly` — the scheduled slow/cross-process replication lane.
+
+### Time control (paused vs real clock)
+
+Time-driven tests should prefer paused time (`tokio::time` with `start_paused`
+and `advance`) or explicit event synchronization over fixed `sleep` race
+windows. The written convention and the `docs/testing/time-control.md` guide are
+added by backlog#1153 infra-4.
 
 ## Flake policy