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/ui/checkbox/checkbox.component.ts

@@ -1,6 +1,11 @@
-import { Component, forwardRef, input } from '@angular/core';
+import { Component, computed, forwardRef, input } from '@angular/core';
 import { ControlValueAccessor, NG_VALUE_ACCESSOR } from '@angular/forms';
 
+// Per-instance fallback ids, so the label's `for` always targets THIS checkbox. The
+// CIBG styled checkbox hides the native input and routes clicks through the label, so a
+// shared/undefined id silently makes every label toggle the first input — hence a default.
+let nextCheckboxId = 0;
+
 /** Atom: a labelled checkbox wired as a form control (ngModel/reactive). Thin
     wrapper over the CIBG Huisstijl `.form-check.styled` checkbox CSS; native
     input for full keyboard + screen-reader support. */
@@ -11,13 +16,13 @@ import { ControlValueAccessor, NG_VALUE_ACCESSOR } from '@angular/forms';
       <input
         class="form-check-input"
         type="checkbox"
-        [id]="checkboxId()"
+        [id]="resolvedId()"
         [checked]="value"
         [disabled]="disabled"
         (change)="onToggle($event)"
         (blur)="onTouched()"
       />
-      <label class="form-check-label" [for]="checkboxId()">{{ label() }}</label>
+      <label class="form-check-label" [for]="resolvedId()">{{ label() }}</label>
     </div>
   `,
   providers: [
@@ -28,6 +33,10 @@ export class CheckboxComponent implements ControlValueAccessor {
   checkboxId = input<string>();
   label = input('');
 
+  /** The caller's id, or a unique fallback — never undefined, so labels never collide. */
+  private autoId = `app-checkbox-${nextCheckboxId++}`;
+  protected resolvedId = computed(() => this.checkboxId() ?? this.autoId);
+
   value = false;
   disabled = false;
   onChange: (v: boolean) => void = () => {};