feat(brief): letter composition + two-person approval (teaching slice)

New `brief` context — a letter-composition feature with a drafter/approver
approval workflow, built as a teaching vertical slice on the repo's existing
FP + Elm + atomic-design patterns (see plan in ~/.claude/plans).

Domain (pure):
- Rich text as a serialisable value tree (placeholders are first-class nodes),
  moved to @shared/kernel/rich-text.ts so the shared editor can use it.
- lintPlaceholders: a pure, total content -> Diagnostic[] linter, derived never stored.
- brief.machine.ts: status sum-type with guarded transitions; frozen-snapshot =
  deep value copy; derived diagnostics/editability. Full specs.

Backend (.NET stub):
- BriefStore + seed, GET/PUT /brief and submit/approve/reject/send endpoints,
  role via X-Role header (mirrors X-Admin), transition + approver!=drafter guards,
  audit logging. Regenerated typed client via gen:api. +6 backend tests.

Seam:
- brief.adapter.ts maps flat wire unions <-> domain discriminated unions at the
  parse boundary (+ spec).

UI (atomic):
- shared atoms: checkbox, placeholder-chip; molecule: rich-text-editor (no-dep
  contenteditable, DOM<->RichTextBlock round-trip tested).
- brief/ui: letter-block, passage-picker, diagnostics-panel, rejection-comments,
  letter-section, letter-composer, letter-preview, brief.page + /brief route.
- Dev-only ?role=drafter|approver toggle + roleInterceptor; dashboard nav link.

Enforcement: @brief/* alias + eslint layer boundary (brief depends only on shared).

Also included (same session):
- Value-object specs (postcode/uren/big-nummer) — closes the "domain must have a spec" gap.
- src/docs/ Storybook MDX foundation pages (atomic design, tokens, FP-in-UI).
- .storybook/tsconfig.json: add @angular/localize to types (Storybook was fully
  broken — $localize unresolved — dev + build).

Verified: 168 FE tests, 68 backend tests, lint/build/check:tokens green,
Storybook boots, end-to-end HTTP smoke (self-approve 403, approver 200, full flow).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/brief/domain/brief.spec.ts

@@ -0,0 +1,49 @@
+import { describe, it, expect } from 'vitest';
+import { Brief, LetterBlock, allDiagnostics, canSubmit, hasBlockingErrors, unresolvedPlaceholders } from './brief';
+import { PlaceholderDef } from './placeholders';
+import { RichTextBlock } from '@shared/kernel/rich-text';
+
+const placeholders: PlaceholderDef[] = [
+  { key: 'naam', label: 'Naam', autoResolvable: true },
+  { key: 'reden', label: 'Reden', autoResolvable: false },
+];
+
+const content = (...keys: string[]): RichTextBlock => ({
+  paragraphs: [{ nodes: keys.map((key) => ({ type: 'placeholder', key })) }],
+});
+
+const passage = (blockId: string, ...keys: string[]): LetterBlock => ({
+  type: 'freeText',
+  blockId,
+  content: content(...keys),
+});
+
+function brief(sections: Brief['sections']): Brief {
+  return { briefId: 'b1', beroep: 'arts', templateId: 't1', placeholders, sections, status: { tag: 'draft' }, drafterId: 'u1' };
+}
+
+describe('brief selectors', () => {
+  it('unresolvedPlaceholders returns deduped manual keys only (auto excluded)', () => {
+    const b = brief([
+      { sectionKey: 's1', title: 'S1', required: true, blocks: [passage('local-1', 'naam', 'reden')] },
+      { sectionKey: 's2', title: 'S2', required: false, blocks: [passage('local-2', 'reden')] },
+    ]);
+    expect(unresolvedPlaceholders(b)).toEqual(['reden']); // 'naam' is auto; 'reden' deduped
+  });
+
+  it('allDiagnostics flattens across sections and blocks', () => {
+    const b = brief([
+      { sectionKey: 's1', title: 'S1', required: true, blocks: [passage('local-1', 'reden', 'onbekend')] },
+    ]);
+    const codes = allDiagnostics(b).map((d) => d.code);
+    expect(codes).toContain('unresolved-at-send'); // reden
+    expect(codes).toContain('unknown-placeholder'); // onbekend
+    expect(hasBlockingErrors(allDiagnostics(b))).toBe(true); // unknown is an error
+  });
+
+  it('canSubmit is false when a required section is empty, true otherwise', () => {
+    expect(canSubmit(brief([{ sectionKey: 's1', title: 'S1', required: true, blocks: [] }]))).toBe(false);
+    expect(canSubmit(brief([{ sectionKey: 's1', title: 'S1', required: false, blocks: [] }]))).toBe(true);
+    expect(canSubmit(brief([{ sectionKey: 's1', title: 'S1', required: true, blocks: [passage('local-1')] }]))).toBe(true);
+  });
+});