feat(fp): brief v3 — besluit-driven guided drafting

Compose the herregistratie letter from the besluit instead of a library hunt:
the behandelaar picks positief/negatief (+ reden-checkboxes for a negatief) and
the kern's standaardteksten follow the selection live.

Front-end (this increment):
- Kern is recomposed reactively from the besluit selection (new BesluitSelected
  machine msg + composeKern); the "Genereer conceptbrief" button is gone. The
  drafter's free text is preserved across a selection change.
- The editor shows only the editable sections; the locked aanhef/slot render in
  the preview, not the authoring surface. Slot is a case-type template section
  (per templateId), documented as such.
- The panel re-seeds from the letter via inferSelection() — the besluit + redenen
  are read back off the kern's passage blocks, so the selection survives reload
  with no new wire fields (derive, don't store).
- letter-section drops the now-redundant per-section passage picker (besluit owns
  standaardteksten); keeps free-text + block edit/move/remove.

Fix: app-checkbox now falls back to a unique per-instance id when checkboxId is
omitted. The CIBG styled checkbox routes clicks through the label, so the shared
id="undefined" made every reason label toggle the first input — the second
checkbox could never be checked. Verified live (Playwright): each reason toggles
independently.

Backend/seam (brief v3 WIP): besluit/reason passage tags on the wire + seed,
carried through the adapter parse boundary.

Specs updated (besluit, brief.machine) and the affected stories re-pointed at the
new API. FE lint + build + 253 vitest specs green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/brief/application/brief.store.ts

@@ -4,6 +4,7 @@ import { RemoteData } from '@shared/application/remote-data';
 import { createStore } from '@shared/application/store';
 import {
   Brief,
+  CaseContext,
   allDiagnostics,
   canSubmit,
   hasBlockingErrors,
@@ -87,6 +88,10 @@ export class BriefStore {
       stays untouched by design). Set from every server view that carries it. */
   readonly orgTemplate = signal<OrgTemplate | null>(null);
 
+  /** The case (zorgverlener + aanvraag) this letter concerns — server-joined context for
+      the behandel scherm header, not letter state. Set from every server view. */
+  readonly caseContext = signal<CaseContext | null>(null);
+
   /** The org logo's content URL for the letterhead, or null when the template has none. */
   readonly logoUrl = computed<string | null>(() => {
     const id = this.orgTemplate()?.logoDocumentId;
@@ -134,6 +139,7 @@ export class BriefStore {
     const r = await this.adapter.load();
     if (r.ok) {
       this.orgTemplate.set(r.value.orgTemplate);
+      this.caseContext.set(r.value.caseContext);
       this.clearHistory();
       this.store.dispatch({ tag: 'BriefLoaded', ...r.value });
     } else {
@@ -212,6 +218,7 @@ export class BriefStore {
     if (r.ok) {
       this.actionState.set({ tag: 'Idle' });
       this.orgTemplate.set(r.value.orgTemplate);
+      this.caseContext.set(r.value.caseContext);
       this.clearHistory();
       this.rejectionSnapshot.set(null);
       this.store.dispatch({ tag: 'BriefLoaded', ...r.value });
@@ -257,6 +264,7 @@ export class BriefStore {
   private applyServerStatus(view: BriefView) {
     // `send` pins the org-template version server-side — mirror whatever came back.
     this.orgTemplate.set(view.orgTemplate);
+    this.caseContext.set(view.caseContext);
     const { brief, decisions } = view;
     const s = brief.status;
     switch (s.tag) {