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/infrastructure/brief.adapter.ts

@@ -7,6 +7,7 @@ import {
   BriefDto,
   BriefStatusDto,
   BriefViewDto,
+  CaseContextDto,
   LetterBlockDto,
   LetterSectionDto,
   LibraryPassageDto,
@@ -19,6 +20,7 @@ import {
   Brief,
   BriefDecisions,
   BriefStatus,
+  CaseContext,
   LetterBlock,
   LetterSection,
   LibraryPassage,
@@ -40,6 +42,7 @@ export interface BriefView {
   readonly availablePassages: LibraryPassage[];
   readonly decisions: BriefDecisions;
   readonly orgTemplate: OrgTemplate;
+  readonly caseContext: CaseContext;
 }
 
 export const BRIEF_LOAD_FAILED = $localize`:@@brief.load.failed:De brief kon niet worden geladen.`;
@@ -249,7 +252,25 @@ function parsePassage(dto: LibraryPassageDto): Result<string, LibraryPassage> {
     content: content.value,
     version: dto.version,
     ...(dto.beroep != null ? { beroep: dto.beroep } : {}),
-    ...(dto.isDefault ? { isDefault: true } : {}),
+    ...(dto.besluit === 'positief' || dto.besluit === 'negatief' ? { besluit: dto.besluit } : {}),
+    ...(dto.reason != null ? { reason: dto.reason } : {}),
+  });
+}
+
+function parseCaseContext(dto: CaseContextDto | undefined): Result<string, CaseContext> {
+  if (
+    typeof dto?.zorgverlenerNaam !== 'string' ||
+    typeof dto.bigNummer !== 'string' ||
+    typeof dto.beroep !== 'string' ||
+    typeof dto.aanvraagReferentie !== 'string'
+  ) {
+    return err('brief-view: missing/invalid case context');
+  }
+  return ok({
+    zorgverlenerNaam: dto.zorgverlenerNaam,
+    bigNummer: dto.bigNummer,
+    beroep: dto.beroep,
+    aanvraagReferentie: dto.aanvraagReferentie,
   });
 }
 
@@ -351,6 +372,8 @@ export function parseBriefView(dto: BriefViewDto): Result<string, BriefView> {
   if (!decisions.ok) return decisions;
   const orgTemplate = parseOrgTemplate(dto.orgTemplate);
   if (!orgTemplate.ok) return orgTemplate;
+  const caseContext = parseCaseContext(dto.caseContext);
+  if (!caseContext.ok) return caseContext;
   const availablePassages: LibraryPassage[] = [];
   for (const p of dto.availablePassages ?? []) {
     const parsed = parsePassage(p);
@@ -362,6 +385,7 @@ export function parseBriefView(dto: BriefViewDto): Result<string, BriefView> {
     availablePassages,
     decisions: decisions.value,
     orgTemplate: orgTemplate.value,
+    caseContext: caseContext.value,
   });
 }