feat(ascii-art): 19 named ASCII art templates with snapshot tests

Per Task 3 of the 2026-09-14 ASCII art upgrade plan.

- New module: src/main/AsciiArt.templates.js exporting ASCII_TEMPLATES
  (19 entries) and getTemplate(name) accessor (returns '' for unknown).
- New test: tests/main/ascii-art.templates.test.js with 19 per-template
  snapshots + unknown-name + keys-match assertions (21 tests total).
- 17 templates verbatim from src/ascii-generator.html:596-626
  (arrow-right, arrow-down, decision, process, flowchart, sequence,
  network, hierarchy, header, note, warning, info, divider, separator,
  banner, checklist + the brief's own TEMPLATE_NAMES order).
- 2 templates verbatim from src/renderer.js:6542-6697
  (progress-bar, table-simple).
- 1 newly authored: arrow-up (completes the arrow triplet; does not
  exist in either source).

Amit Haridas

tests/main/ascii-art.templates.test.js

@@ -0,0 +1,45 @@
+/**
+ * @jest-environment node
+ *
+ * Per-template snapshot tests. Templates are static strings; the snapshot
+ * catches accidental edits during refactors.
+ */
+const AsciiArtTemplates = require('../../src/main/AsciiArt.templates');
+
+const TEMPLATE_NAMES = [
+  'arrow-right',
+  'arrow-down',
+  'arrow-up',
+  'decision',
+  'process',
+  'flowchart',
+  'sequence',
+  'network',
+  'hierarchy',
+  'header',
+  'note',
+  'warning',
+  'info',
+  'divider',
+  'separator',
+  'banner',
+  'checklist',
+  'progress-bar',
+  'table-simple',
+];
+
+describe('AsciiArt.templates', () => {
+  test.each(TEMPLATE_NAMES)('%s', (name) => {
+    expect(AsciiArtTemplates.getTemplate(name)).toMatchSnapshot();
+  });
+
+  test('returns empty string for unknown name', () => {
+    expect(AsciiArtTemplates.getTemplate('nope')).toBe('');
+  });
+
+  test('ASCII_TEMPLATES object exposes all 19 names', () => {
+    expect(Object.keys(AsciiArtTemplates.ASCII_TEMPLATES).sort()).toEqual(
+      TEMPLATE_NAMES.slice().sort()
+    );
+  });
+});