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/ui/letter-editor/letter-editor.component.ts

@@ -0,0 +1,40 @@
+import { Component, computed, input, output } from '@angular/core';
+import { PlaceholderOption } from '@shared/ui/rich-text-editor/rich-text-editor.component';
+import { Brief } from '@brief/domain/brief';
+import { BriefMsg } from '@brief/domain/brief.machine';
+import { LetterSectionComponent } from '@brief/ui/letter-section/letter-section.component';
+
+/** Organism: the lean authoring surface — just the editable letter sections and their
+    add/edit controls, no letterhead/signature/footer/zoom. The full rendered letter
+    (including the locked aanhef/slot) lives in the preview modal (see behandel-scherm),
+    so the drafter stays focused on composing the body they actually own. */
+@Component({
+  selector: 'app-letter-editor',
+  imports: [LetterSectionComponent],
+  styles: [
+    `
+      :host {
+        display: grid;
+        gap: var(--rhc-space-max-xl);
+      }
+    `,
+  ],
+  template: `
+    @for (section of editableSections(); track section.sectionKey) {
+      <app-letter-section
+        [section]="section"
+        [placeholders]="placeholders()"
+        [editable]="true"
+        (edit)="edit.emit($event)"
+      />
+    }
+  `,
+})
+export class LetterEditorComponent {
+  brief = input.required<Brief>();
+  placeholders = input<readonly PlaceholderOption[]>([]);
+  edit = output<BriefMsg>();
+
+  /** Only unlocked sections are authored here; locked aanhef/slot appear in the preview. */
+  protected editableSections = computed(() => this.brief().sections.filter((s) => !s.locked));
+}

src/app/brief/ui/letter-editor/letter-editor.stories.ts

@@ -0,0 +1,62 @@
+import type { Meta, StoryObj } from '@storybook/angular';
+import { Brief } from '@brief/domain/brief';
+import { LetterEditorComponent } from './letter-editor.component';
+
+const brief: Brief = {
+  briefId: 'b1',
+  beroep: 'arts',
+  templateId: 't1',
+  drafterId: 'demo-drafter',
+  status: { tag: 'draft' },
+  placeholders: [{ key: 'reden_besluit', label: 'Reden besluit', autoResolvable: false }],
+  sections: [
+    {
+      sectionKey: 'aanhef',
+      title: 'Aanhef',
+      required: true,
+      locked: true,
+      blocks: [
+        {
+          type: 'freeText',
+          blockId: 'aanhef-1',
+          content: { paragraphs: [{ nodes: [{ type: 'text', text: 'Geachte heer/mevrouw,' }] }] },
+        },
+      ],
+    },
+    {
+      sectionKey: 'kern',
+      title: 'Kern van het besluit',
+      required: true,
+      locked: false,
+      blocks: [
+        {
+          type: 'freeText',
+          blockId: 'kern-1',
+          content: { paragraphs: [{ nodes: [{ type: 'text', text: 'Wij hebben besloten...' }] }] },
+        },
+      ],
+    },
+  ],
+};
+
+const meta: Meta<LetterEditorComponent> = {
+  title: 'Domein/Brief/Letter Editor',
+  component: LetterEditorComponent,
+  args: { brief, placeholders: [] },
+};
+export default meta;
+type Story = StoryObj<LetterEditorComponent>;
+
+/** The lean authoring surface: only the editable sections (the kern); the locked
+    aanhef/slot are hidden here and appear only in the preview. */
+export const Default: Story = {};
+
+/** Empty kern: shows the empty-state hint plus the free-text action, no starter content. */
+export const EmptyKern: Story = {
+  args: {
+    brief: {
+      ...brief,
+      sections: brief.sections.map((s) => (s.sectionKey === 'kern' ? { ...s, blocks: [] } : s)),
+    },
+  },
+};