test(frontend): replay real messages instead of hand-built state literals (WP-70)

Every machine spec redefined its own throwaway fixture helper (editing1/2/3,
editingWith), hardcoding fields like errors: {} that assert against shapes
the reducer may never actually produce. given(reduce, initial)(...msgs)
(libs/shared/src/testing/machine.ts) replaces them by replaying real Msgs
through the real reduce, so a fixture is provably reachable. Adds the same
idiom for value objects (unwrapOk) and RemoteData (loading/success/failure),
plus intake.acceptance.spec.ts as a worked full-journey example.

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

libs/shared/src/testing/machine.ts

@@ -0,0 +1,15 @@
+/**
+ * Test-only DSL for the Elm-store idiom (CLAUDE.md #3). A fixture is built by
+ * replaying real `Msg`s through the real `reduce` — never by hand-assembling a
+ * state object field-by-field. That closes off illegal states the reducer would
+ * never actually produce: if a spec can't reach a state via messages, it can't
+ * assert on it either.
+ *
+ * `given(reduce, initial)` partially applies a machine's reducer + starting
+ * state; the result is a variadic replay function a spec calls with the exact
+ * message sequence a real user/flow would send.
+ */
+export const given =
+  <S, M>(reduce: (s: S, m: M) => S, initial: S) =>
+  (...msgs: M[]): S =>
+    msgs.reduce(reduce, initial);

libs/shared/src/testing/remote-data.ts

@@ -0,0 +1,13 @@
+import { RemoteData } from '@shared/application/remote-data';
+
+/**
+ * Test-only constructors for `RemoteData` — one line per variant, so a spec
+ * builds fixtures through the same tagged-union shape production code renders
+ * (`foldRemote`/`<app-async>`), never a hand-rolled literal that could drift
+ * from the real type.
+ */
+export const loading = <E = never, T = never>(): RemoteData<E, T> => ({ tag: 'Loading' });
+
+export const success = <T, E = never>(value: T): RemoteData<E, T> => ({ tag: 'Success', value });
+
+export const failure = <E, T = never>(error: E): RemoteData<E, T> => ({ tag: 'Failure', error });

libs/shared/src/testing/value-object.ts

@@ -0,0 +1,14 @@
+import { Result } from '@shared/kernel/fp';
+
+/**
+ * Unwrap a `Result` produced by a REAL `parse*` value-object parser, throwing
+ * if it isn't `ok`. This is the only sanctioned way for a spec to obtain a
+ * branded value-object type — it closes off the `'garbage' as Postcode` cast
+ * route, since the only door to the branded type is the parser itself.
+ */
+export function unwrapOk<E, T>(result: Result<E, T>): T {
+  if (!result.ok) {
+    throw new Error(`unwrapOk: expected ok, got error: ${JSON.stringify(result.error)}`);
+  }
+  return result.value;
+}