test: cover the text editor engine and FreeText flattening

Adds a node-side harness that drives the engine wasm directly, fixtures
that build PDFs in-memory, and cases for font fallback and flattening.
The engine loader now tolerates a runtime without import.meta.resolve so
the suite can load it outside a browser.

src/tests/editcore-smoke.test.ts

@@ -0,0 +1,80 @@
+// @vitest-environment node
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import {
+  createEngine,
+  modelOf,
+  paragraphText,
+  allText,
+  runsFrom,
+  type Engine,
+} from './helpers/editcore-harness';
+import { twoParagraphPdf, A_TEXT, B_TEXT } from './helpers/editcore-fixtures';
+
+describe('vendored editcore engine', () => {
+  let engine: Engine;
+
+  beforeAll(async () => {
+    engine = await createEngine();
+  });
+
+  afterAll(() => {
+    engine.close();
+  });
+
+  it('loads the vendored wasm and builds a page model', () => {
+    const model = modelOf(engine, twoParagraphPdf());
+    expect(model).toHaveLength(2);
+    expect(paragraphText(model[0])).toBe(A_TEXT);
+    expect(paragraphText(model[1])).toBe(B_TEXT);
+  });
+
+  it('reports paragraphs as editable', () => {
+    for (const p of modelOf(engine, twoParagraphPdf())) {
+      expect(p.editable).toBe(true);
+      expect(p.lockReason).toBe(0);
+    }
+  });
+
+  it('commits an edit into the page model', () => {
+    const model = modelOf(engine, twoParagraphPdf());
+    const target = model[0];
+    engine.commitParagraph(
+      target.id,
+      runsFrom(target, paragraphText(target).replace('quick', 'quicks')),
+      target.format
+    );
+    const after = engine.buildModel();
+    expect(paragraphText(after[0])).toBe(A_TEXT.replace('quick', 'quicks'));
+    expect(paragraphText(after[1])).toBe(B_TEXT);
+  });
+
+  it('splices an edit back into a re-openable document', async () => {
+    const model = modelOf(engine, twoParagraphPdf());
+    const target = model[0];
+    engine.commitParagraph(
+      target.id,
+      runsFrom(target, paragraphText(target).replace('quick', 'quicks')),
+      target.format
+    );
+
+    const saved = await engine.saveSpliced();
+    expect(saved).toBeTruthy();
+    expect(saved!.length).toBeGreaterThan(0);
+    expect(new TextDecoder().decode(saved!.subarray(0, 5))).toBe('%PDF-');
+
+    const text = allText(modelOf(engine, saved!));
+    expect(text).toContain('The quicks brown fox');
+    expect(text).toContain(B_TEXT);
+  });
+
+  it('splices a deletion back into the document', async () => {
+    const model = modelOf(engine, twoParagraphPdf());
+    engine.deleteParagraph(model[0].id);
+    const saved = await engine.saveSpliced();
+    expect(saved).toBeTruthy();
+
+    const text = allText(modelOf(engine, saved!));
+    expect(text).toContain(B_TEXT);
+    expect(text).not.toContain('The quick brown fox');
+  });
+});