Implement M1 authoring loop: library, editor, live preview, persistence

M1 MVP from docs/IMPLEMENTATION-PLAN.md, with the /alignment pass applied.

- core: Snippet model + factory; prepareSpecForRender (copy-not-mutate); per-theme Vega chart config
- state/orchestration: SnippetStore with debounced auto-save; IndexedDB adapter + read-time migration; startup hydration + write-through persistence
- ui: SnippetLibrary, SpecEditor (Monaco edcore.main — full editor features, JSON-only languages), LivePreview
- build: Monaco/Vega manual chunks; raised PWA precache ceiling
- alignment: flush a valid draft on snippet switch (+regression tests); TODO breadcrumbs for the preview render race and the window.confirm delete
- housekeeping: gitignore .claude/projects/

src/core/snippet.test.ts

@@ -0,0 +1,74 @@
+import { describe, expect, test } from 'vitest';
+import {
+  CURRENT_SNIPPET_VERSION,
+  createSnippet,
+  generateSnippetName,
+  hasUnpublishedChanges,
+  SAMPLE_SPEC,
+  sampleSpecText,
+} from './snippet';
+
+describe('createSnippet', () => {
+  test('stamps current version, equal timestamps, and the sample template', () => {
+    const now = new Date('2026-06-04T14:30:07.000Z');
+    const s = createSnippet({ now, id: 'fixed-id' });
+
+    expect(s.id).toBe('fixed-id');
+    expect(s.version).toBe(CURRENT_SNIPPET_VERSION);
+    expect(s.created).toBe(now.toISOString());
+    expect(s.modified).toBe(s.created);
+    expect(s.spec).toBe(sampleSpecText());
+    // draft starts identical to published — nothing to publish yet.
+    expect(s.draftSpec).toBe(s.spec);
+    expect(s.comment).toBe('');
+    expect(s.tags).toEqual([]);
+    expect(s.datasetRefs).toEqual([]);
+    expect(s.meta).toEqual({});
+  });
+
+  test('generates a unique id by default', () => {
+    const a = createSnippet();
+    const b = createSnippet();
+    expect(a.id).not.toBe(b.id);
+  });
+
+  test('accepts name and spec overrides', () => {
+    const s = createSnippet({ name: 'Custom', spec: '{"mark":"point"}' });
+    expect(s.name).toBe('Custom');
+    expect(s.spec).toBe('{"mark":"point"}');
+    expect(s.draftSpec).toBe('{"mark":"point"}');
+  });
+});
+
+describe('the sample template', () => {
+  test('is valid JSON that round-trips', () => {
+    expect(JSON.parse(sampleSpecText())).toEqual(SAMPLE_SPEC);
+  });
+
+  test('is a bar chart with inline data', () => {
+    expect(SAMPLE_SPEC.mark).toBe('bar');
+    expect(SAMPLE_SPEC.data.values.length).toBeGreaterThan(0);
+  });
+});
+
+describe('generateSnippetName', () => {
+  test('formats as "Snippet YYYY-MM-DD HH:MM:SS" with zero-padding', () => {
+    // Construct via local-time parts so the test is timezone-independent.
+    const now = new Date(2026, 0, 5, 9, 7, 3); // 2026-01-05 09:07:03 local
+    expect(generateSnippetName(now)).toBe('Snippet 2026-01-05 09:07:03');
+  });
+
+  test('differs second-to-second so quick successive creates stay distinct', () => {
+    const a = generateSnippetName(new Date(2026, 5, 4, 14, 30, 7));
+    const b = generateSnippetName(new Date(2026, 5, 4, 14, 30, 8));
+    expect(a).not.toBe(b);
+  });
+});
+
+describe('hasUnpublishedChanges', () => {
+  test('false when draft matches published, true once draft diverges', () => {
+    const s = createSnippet({ now: new Date('2026-06-04T00:00:00Z') });
+    expect(hasUnpublishedChanges(s)).toBe(false);
+    expect(hasUnpublishedChanges({ ...s, draftSpec: s.spec + ' ' })).toBe(true);
+  });
+});