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/README.md

@@ -46,7 +46,7 @@ for its existing violations, so every WP ends green.
 | [WP-04](WP-04-ui-not-infrastructure.md) | Boundaries II: `ui ↛ infrastructure` + showcase sanction     | 0 · gates                   | done   |
 | [WP-05](WP-05-parse-boundaries.md)      | Parse-don't-validate closure + MDX                           | 1 · FP/DDD                  | done   |
 | [WP-06](WP-06-typed-async.md)           | Generic async template contexts — kill `$any()`              | 1 · FP/DDD                  | done   |
-| [WP-07](WP-07-brief-idioms.md)          | Brief on the shared idioms + RemoteData MDX                  | 1 · FP/DDD                  | todo   |
+| [WP-07](WP-07-brief-idioms.md)          | Brief on the shared idioms + RemoteData MDX                  | 1 · FP/DDD                  | done   |
 | [WP-08](WP-08-store-idiom.md)           | One store idiom + machine naming + TEA MDX                   | 1 · FP/DDD                  | todo   |
 | [WP-09](WP-09-pure-logic.md)            | Pure-logic closure: dates + missing command specs            | 1 · FP/DDD                  | todo   |
 | [WP-10](WP-10-button-fidelity.md)       | CIBG button fidelity                                         | 2 · CIBG                    | todo   |