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/shared/infrastructure/api-client.ts

@@ -1608,6 +1608,7 @@ export interface BriefViewDto {
     availablePassages?: LibraryPassageDto[] | undefined;
     decisions?: BriefDecisionsDto;
     orgTemplate?: OrgTemplateDto;
+    caseContext?: CaseContextDto;
 }
 
 export interface BrpAddressDto {
@@ -1615,6 +1616,13 @@ export interface BrpAddressDto {
     adres?: AdresDto;
 }
 
+export interface CaseContextDto {
+    zorgverlenerNaam?: string | undefined;
+    bigNummer?: string | undefined;
+    beroep?: string | undefined;
+    aanvraagReferentie?: string | undefined;
+}
+
 export interface ChangeRequestRequest {
     straat?: string | undefined;
     postcode?: string | undefined;
@@ -1713,6 +1721,8 @@ export interface LibraryPassageDto {
     version?: number;
     beroep?: string | undefined;
     isDefault?: boolean;
+    besluit?: string | undefined;
+    reason?: string | undefined;
 }
 
 export interface ManualDiplomaPolicyDto {