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/rendering.test.ts

@@ -0,0 +1,42 @@
+import { describe, expect, test } from 'vitest';
+import { escapeVegaField, prepareSpecForRender } from './rendering';
+
+describe('prepareSpecForRender', () => {
+  test('returns a deep copy, never the same reference', () => {
+    const spec = { mark: 'bar', encoding: { x: { field: 'a' } } };
+    const out = prepareSpecForRender(spec);
+    expect(out).not.toBe(spec);
+    expect(out.encoding).not.toBe(spec.encoding);
+    expect(out).toEqual(spec);
+  });
+
+  test('never mutates the input spec (copy-not-mutate invariant)', () => {
+    const spec = {
+      data: { values: [{ a: 1 }] },
+      mark: 'bar',
+      encoding: { x: { field: 'a', type: 'quantitative' } },
+    };
+    const before = structuredClone(spec);
+    const out = prepareSpecForRender(spec, { fitMode: 'width' });
+
+    // Mutating the output must not touch the input.
+    (out as { mark: string }).mark = 'point';
+    expect(spec).toEqual(before);
+  });
+
+  test('M1 is a faithful pass-through of the spec content', () => {
+    const spec = { $schema: 'x', mark: 'line', width: 200, height: 100 };
+    expect(prepareSpecForRender(spec)).toEqual(spec);
+  });
+});
+
+describe('escapeVegaField', () => {
+  test('escapes dots and brackets that VL treats as accessors', () => {
+    expect(escapeVegaField('user.age')).toBe('user\\.age');
+    expect(escapeVegaField('a[0]')).toBe('a\\[0\\]');
+  });
+
+  test('leaves plain field names untouched', () => {
+    expect(escapeVegaField('category')).toBe('category');
+  });
+});