feat(vom): support canvas reading

apps/extension/test-fixtures/rendered-surfaces/canvas-input.html

@@ -0,0 +1,20 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1" />
+    <title>Canvas to standard input fixture</title>
+    <style>
+      body { padding: 16px; font: 14px system-ui, sans-serif; }
+      iframe { width: 300px; height: 220px; border: 2px solid #5d6d7e; }
+    </style>
+  </head>
+  <body>
+    <h1>Canvas to standard input fixture</h1>
+    <p>Select row 3 in the visual-only Canvas, then fill the exposed standard textbox.</p>
+    <iframe
+      title="cross-origin Canvas editor"
+      src="http://localhost:4173/frame.html?label=canvas-input-grid&interactive=1&editor=1"
+    ></iframe>
+  </body>
+</html>