feat(fp): WP-07 — brief on the shared idioms + RemoteData MDX

Collapse brief.store's busy signal + nullable lastError into one Idle |
Busy | Failed union (saveState gets matching tag-object style), and route
brief.page's load through RemoteData + <app-async> instead of a hand-rolled
@switch, via a BriefStore.remoteData projection of the machine's existing
loading/failed tags -- the machine keeps owning the letter's own status
lifecycle untouched. New brief.store.spec.ts covers the Busy->Idle/Failed
transitions; new Foundations/RemoteData & Async MDX page documents the
pattern and the WP-06 typed-loaded-slot fallback. Deviation from the
original plan recorded in the WP file.

docs/backlog/WP-07-brief-idioms.md

@@ -1,6 +1,6 @@
 # WP-07 — Brief on the shared idioms + RemoteData MDX
 
-Status: todo
+Status: done
 Phase: 1 — FP/DDD core
 Depends on: WP-06 (typed `<app-async>`)
 
@@ -50,15 +50,53 @@ bypassing the shared molecule.
 
 ## Acceptance criteria
 
-- [ ] No boolean-plus-error signal pairs in `brief/`.
-- [ ] `/brief` renders all four async states (check with `?scenario=slow|empty|error`).
-- [ ] Specs cover the transition union (Busy→Failed, Busy→Idle).
-- [ ] MDX renders under Foundations.
+- [x] No boolean-plus-error signal pairs in `brief/`.
+- [x] `/brief` renders all four async states (checked with `?scenario=slow|error`; see
+      Deviation for why `empty` isn't meaningful here).
+- [x] Specs cover the transition union (Busy→Failed, Busy→Idle) — `brief.store.spec.ts`
+      (new).
+- [x] MDX renders under Foundations.
 
 ## Verification
 
-GREEN + `npm run test-storybook:ci`. Manual: `npm start` → `/brief` with
-`?scenario=slow`, `?scenario=error`; exercise autosave + submit + rejection flow.
+GREEN + `npm run test-storybook:ci` (197 unit tests, 137 Storybook/a11y — both up from
+WP-06's baseline by the new store spec). Manual smoke via a running `docker compose`
+stack + Playwright: `/brief` normal load, `?scenario=slow` (spinner), `?scenario=error`
+(failure alert + working retry), and `/brief?role=approver` — all with no console errors.
+
+## Deviation from the original plan
+
+**The machine's `loading`/`failed` tags were NOT moved out of `BriefState`.** The
+Decisions block hedges this ("only if they purely mirror the fetch") — they do, but
+removing them turns out to need more than a re-type: `createStore(initial, reduce)`
+requires a concrete `initial: BriefState` value, and once `loading`/`failed` are gone
+there is no state left to represent "not loaded yet" without inventing a second wrapping
+layer (the store's top-level signal would need to become `RemoteData<Err, LoadedState>`
+directly, with the machine's `reduce` only invoked inside the `Success` branch — a
+different wiring shape from every other machine in the app, and a ~250-line ripple
+through `brief.machine.spec.ts`). That redesign is a bigger, riskier change than this WP's
+"re-type, don't restructure" framing calls for.
+
+Instead, `BriefStore.remoteData` **projects** the existing machine model onto
+`RemoteData<Error | undefined, LoadedBriefState>` (`loading`→`Loading`, `failed`→
+`Failure`, `loaded`→`Success`), and `brief.page.ts` renders that projection through
+`<app-async>`. This satisfies the actual goal (the load lifecycle renders through the
+shared molecule, not a hand-rolled `@switch`) without touching `brief.machine.ts` or its
+spec at all — `BriefState` keeps its three tags exactly as they were. The seam holds:
+`RemoteData` still owns "is the fetch done", the machine still owns "what is the letter
+doing" (draft/submitted/approved/rejected/sent) once loaded.
+
+**`?scenario=empty` doesn't apply to `/brief`.** It rewrites the HTTP body to `[]`, which
+fails `parseBriefView`'s `!dto.brief` check — the same as any malformed response, so it
+surfaces as a `Failure`, not an `Empty`. A single-letter GET has no meaningful "empty"
+state (unlike a list endpoint), so this isn't a gap — `AsyncComponent`'s `Empty` branch
+simply never fires for this resource, by construction (no `isEmpty` input is passed).
+
+**Reused the WP-06 fallback for the loaded slot.** `<ng-template appAsyncLoaded>` can't
+type `let-s` to the loaded value for the same structural reason WP-06 documented
+(a directive's generic can't inherit from a sibling `[data]` input) — `brief.page.ts` adds
+a `loaded` computed and narrows with `@if (loaded(); as s)`, matching
+`dashboard.page.ts`/`registration-detail.page.ts`.
 
 ## Out of scope
 
@@ -67,4 +105,11 @@ Brief component stories (WP-15); machine renaming conventions (WP-08).
 ## Risks
 
 Autosave (debounced) interplay with the new transition union — flush ordering must stay
-as-is; the machine spec pins it.
+as-is; `brief.store.spec.ts`'s Busy→Idle/Failed tests exercise `transition()`, which
+still calls `flushSave()` before the server action exactly as before. One subtle,
+pre-existing edge case changed slightly: if a debounced autosave fails mid-transition
+(setting the error) and the transition's own server action then succeeds, the original
+code left the stale autosave error visible (it only cleared `lastError` at the very start
+of `transition()`/`resetDemo()`); the re-typed version now clears it on that same
+successful end, since `actionState` only holds one current value. Judged an acceptable,
+arguably-corrective difference, not a behavior this WP needed to preserve.