chore: only include visible iframes, keep frame locators in own array (#60)

As discussed:

- hides invisible frames from snapshot
- keep our own frame locator array, so we don't rely on `page.frames()`
ordering to be stable

src/tools/common.ts

@@ -41,7 +41,7 @@ export const navigate: ToolFactory = snapshot => ({
     // Cap load event to 5 seconds, the page is operational at this point.
     await page.waitForLoadState('load', { timeout: 5000 }).catch(() => {});
     if (snapshot)
-      return captureAriaSnapshot(page);
+      return captureAriaSnapshot(context);
     return {
       content: [{
         type: 'text',