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/setup.ts

@@ -13,7 +13,10 @@ if (typeof globalThis.DOMMatrix === 'undefined') {
   globalThis.DOMMatrix = TestDOMMatrix as unknown as typeof DOMMatrix;
 }
 
+const hasDom = typeof window !== 'undefined';
+
 afterEach(() => {
+  if (!hasDom) return;
   document.body.innerHTML = '';
   document.head.innerHTML = '';
 });
@@ -24,19 +27,21 @@ global.ResizeObserver = vi.fn().mockImplementation(() => ({
   disconnect: vi.fn(),
 }));
 
-Object.defineProperty(window, 'matchMedia', {
-  writable: true,
-  value: vi.fn().mockImplementation((query) => ({
-    matches: false,
-    media: query,
-    onchange: null,
-    addListener: vi.fn(),
-    removeListener: vi.fn(),
-    addEventListener: vi.fn(),
-    removeEventListener: vi.fn(),
-    dispatchEvent: vi.fn(),
-  })),
-});
+if (hasDom) {
+  Object.defineProperty(window, 'matchMedia', {
+    writable: true,
+    value: vi.fn().mockImplementation((query) => ({
+      matches: false,
+      media: query,
+      onchange: null,
+      addListener: vi.fn(),
+      removeListener: vi.fn(),
+      addEventListener: vi.fn(),
+      removeEventListener: vi.fn(),
+      dispatchEvent: vi.fn(),
+    })),
+  });
+}
 
 global.IntersectionObserver = vi.fn().mockImplementation(() => ({
   observe: vi.fn(),