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.spec.ts

@@ -1,7 +1,7 @@
 import { TestBed } from '@angular/core/testing';
 import { describe, it, expect, vi, afterEach } from 'vitest';
 import { Result } from '@shared/kernel/fp';
-import { Brief, BriefDecisions, LetterBlock } from '@brief/domain/brief';
+import { Brief, BriefDecisions, CaseContext, LetterBlock } from '@brief/domain/brief';
 import { OrgTemplate } from '@brief/domain/org-template';
 import { BriefAdapter, BriefView } from '@brief/infrastructure/brief.adapter';
 import { LetterPreviewAdapter } from '@brief/infrastructure/letter-preview.adapter';
@@ -37,7 +37,14 @@ const orgTemplate: OrgTemplate = {
   version: 1,
 };
 
-const view: BriefView = { brief, availablePassages: [], decisions, orgTemplate };
+const caseContext: CaseContext = {
+  zorgverlenerNaam: 'Dr. A. (Anna) de Vries',
+  bigNummer: '19012345601',
+  beroep: 'arts',
+  aanvraagReferentie: 'HER-2026-000842',
+};
+
+const view: BriefView = { brief, availablePassages: [], decisions, orgTemplate, caseContext };
 
 function setup(adapter: Partial<BriefAdapter>): BriefStore {
   TestBed.configureTestingModule({ providers: [{ provide: BriefAdapter, useValue: adapter }] });