📝 Require labeled AAA blocks in tests

Rule in CONTRIBUTING.md; decision, why and rejected unlabeled
ordering in development/testing.md.

development/testing.md

@@ -36,6 +36,33 @@ The runner is `node --test --strip-types "src/**/*.test.ts"` and the tiers are i
 build step, and the runner relies on the `.ts` import-extension convention (see
 [tooling.md](./tooling.md#source-imports-use-ts-extensions)).
 
+## AAA ordering
+
+The rule is in
+[CONTRIBUTING.md § Testing discipline (type-driven)](../CONTRIBUTING.md#testing-discipline-type-driven).
+
+#### Decision (2026-09)
+
+Test bodies read arrange → act → assert: inputs (the factory) set up first, the
+subject exercised once from them, all checks last — types then runtime. The
+blocks are labeled with `// Arrange` / `// Act` / `// Assert` comments and
+separated by a blank line; an empty block drops its label.
+
+#### Why
+
+- Interleaved setup/checks hide what runs vs. what is observed; the eye
+  re-reads the block to find the seams.
+- A factory built mid-test invites a second throwaway call of the subject;
+  arranging it once makes the positive construction and the negative
+  `Parameters<…>` check share one source of truth.
+- Labels make the seams explicit, not inferred — grep-able and reviewable
+  without reading the statements.
+
+#### Rejected
+
+- Unlabeled ordering (bare blank lines): the seams still have to be found by
+  reading; the labels cost nothing.
+
 ## Known issues
 
 - The type-aware linter misidentifies `expectTypeOf()` as a floating promise, so