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);