feat: add markdown composer backend foundation

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/renderer/markdownEditor.test.ts

@@ -0,0 +1,73 @@
+import { describe, expect, test } from 'bun:test';
+import { createHeadlessEditor } from '@lexical/headless';
+import { $convertFromMarkdownString, $convertToMarkdownString } from '@lexical/markdown';
+
+import {
+  inspectMarkdownPaste,
+  markdownEditorNamespace,
+  markdownEditorNodes,
+  markdownEditorTransformers,
+} from '@renderer/lib/markdownEditor';
+
+function roundTripMarkdown(markdown: string): string {
+  const editor = createHeadlessEditor({
+    namespace: markdownEditorNamespace,
+    nodes: [...markdownEditorNodes],
+    onError(error) {
+      throw error;
+    },
+  });
+
+  editor.update(() => {
+    $convertFromMarkdownString(markdown, [...markdownEditorTransformers], undefined, true, false);
+  }, { discrete: true });
+
+  let result = '';
+  editor.getEditorState().read(() => {
+    result = $convertToMarkdownString([...markdownEditorTransformers], undefined, true);
+  });
+
+  return result;
+}
+
+describe('markdown editor contract', () => {
+  test('round-trips markdown structures with the chosen node set', () => {
+    const markdown = [
+      '# Release notes',
+      '',
+      '- item one',
+      '- item two',
+      '',
+      '> keep the markdown contract stable',
+      '',
+      '```ts',
+      'const answer = 42;',
+      '```',
+      '',
+      '[Docs](https://example.com)',
+    ].join('\n');
+
+    const roundTripped = roundTripMarkdown(markdown);
+
+    expect(roundTripped).toContain('# Release notes');
+    expect(roundTripped).toContain('- item one');
+    expect(roundTripped).toContain('```ts');
+    expect(roundTripped).toContain('[Docs](https://example.com)');
+  });
+
+  test('classifies fenced code as markdown paste', () => {
+    expect(inspectMarkdownPaste('```ts\nconst answer = 42;\n```')).toEqual({
+      normalizedText: '```ts\nconst answer = 42;\n```',
+      shouldImportMarkdown: true,
+      reason: 'fenced-code',
+    });
+  });
+
+  test('keeps plain prose on the plain-text path', () => {
+    expect(inspectMarkdownPaste('Just a normal sentence about the release plan.')).toEqual({
+      normalizedText: 'Just a normal sentence about the release plan.',
+      shouldImportMarkdown: false,
+      reason: 'plain-text',
+    });
+  });
+});