docs: ADR-0006 test-data builders, close out WP-70

Writes up the principle behind WP-70's three tracks ("build test data
through the same door production code uses") as ADR-0006, with a decision
table for which fixture idiom fits which test type. Updates the
test-strategy skill (adds the Fixtures rule, fixes its stale pre-monorepo
src/app/... worked-example paths) and the shared Storybook testing.mdx page
to match. Closes WP-70 with the signatures/counts as actually shipped.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

libs/shared/docs/testing.mdx

@@ -63,9 +63,38 @@ expect(parseBrpAddress(null).ok).toBe(false);
 expect(parseBrpAddress({}).ok).toBe(false); // missing required field
 ```
 
-Elm-style machines test the pure `reduce` with inline state fixtures — no Angular
+Elm-style machines test the pure `reduce` — no Angular
 (`registratie/domain/registratie-wizard.machine.spec.ts`).
 
+## Fixtures: build test data through the production door
+
+A fixture is not a shortcut around the domain — it's the domain's own construction path, run
+once for the test. [ADR-0006](../../../docs/reference/architecture/0006-test-data-builders.md)
+covers this in full (with a backend example too); the frontend idiom is one combinator,
+`given` (`libs/shared/src/testing/machine.ts`):
+
+```ts
+export const given =
+  <S, M>(reduce: (s: S, m: M) => S, initial: S) =>
+  (...msgs: M[]): S =>
+    msgs.reduce(reduce, initial);
+
+export const givenIntake = given(reduce, initial); // per-context wrapper, pure TS
+```
+
+A machine spec replays real `Msg`s instead of hand-writing a `State` literal — so a fixture
+can only ever be a state the real reducer actually produces:
+
+```ts
+const atStep3 = givenIntake(Start(), SetUren('1200'), Next(), SetDiplomaHerkomst('NL'), Next());
+```
+
+The same rule extends to value objects (`unwrapOk(parseX(raw))` instead of a cast) and to
+`RemoteData` (`loading()` / `success(v)` / `failure(e)` in
+`libs/shared/src/testing/{value-object,remote-data}.ts` instead of a redefined-per-file
+literal). **Never** a `.withX().withY()` builder over an open constructor — that just
+re-opens whatever illegal state the domain closed.
+
 ## UI = Storybook, not heavy component tests
 
 `@storybook/addon-a11y` runs the `wcag2a/2aa/21a/21aa` rule sets on **every** story;