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>

.claude/skills/test-strategy/SKILL.md

@@ -22,6 +22,13 @@ No `TestBed` for domain. Never assert on user-facing copy.
   only for wiring axe can't see.
 - **Never assert on `$localize` copy.** It changes per locale/edit — assert on the
   `Result`, the value object, or the message id.
+- **Fixtures go through the production door, never a hand-built literal** (ADR-0006).
+  Replay real `Msg`s through the real `reduce` (`given(reduce, initial)(...msgs)`) for a
+  state machine; `unwrapOk(parseX(raw))` for a value object; a type-state builder
+  (`Given.Concept().Submitted()...`) for a backend aggregate with an ordered lifecycle.
+  The one deliberate exception is a trust-boundary `parse*` spec, below — there the fixture
+  must be a raw, possibly-malformed literal, because the test's whole point is "what if this
+  shape is wrong." See ADR-0006's decision table for which idiom fits which test type.
 
 ## Skeleton
 
@@ -54,9 +61,13 @@ describe('parseThing', () => {
 
 ## Worked examples
 
-- `src/app/registratie/domain/value-objects/postcode.spec.ts` — parser style.
-- `src/app/registratie/infrastructure/brp.adapter.spec.ts` — trust boundary (`null`/`{}`).
-- `src/app/registratie/domain/registratie-wizard.machine.spec.ts` — pure reducer.
+- `apps/ssp/src/app/registratie/domain/value-objects/postcode.spec.ts` — parser style.
+- `apps/ssp/src/app/registratie/infrastructure/brp.adapter.spec.ts` — trust boundary (`null`/`{}`).
+- `apps/ssp/src/app/registratie/domain/registratie-wizard.machine.spec.ts` — pure reducer.
+- `libs/shared/src/testing/{machine,remote-data,value-object}.ts` — the shared fixture
+  helpers (ADR-0006); `apps/ssp/src/app/herregistratie/domain/intake.testing.ts` — a
+  per-context wrapper (`givenIntake = given(reduce, initial)`); `intake.acceptance.spec.ts`
+  — a full journey expressed as one replayed message sequence.
 
 ## Verify