Phase 2: Enhanced browser — dialog handling, upload, state checks, snapshots

- CircularBuffer O(1) ring buffer for console/network/dialog (was O(n) array+shift)
- Async buffer flush with Bun.write() (was appendFileSync)
- Dialog auto-accept/dismiss with buffer + prompt text support
- File upload command (upload <sel> <file...>)
- Element state checks (is visible/hidden/enabled/disabled/checked/editable/focused)
- Annotated screenshots with ref labels overlaid (-a flag)
- Snapshot diffing against previous snapshot (-D flag)
- Cursor-interactive element scan for non-ARIA clickables (-C flag)
- Snapshot scoping depth limit (-d N flag)
- Health check with page.evaluate + 2s timeout
- Playwright error wrapping — actionable messages for AI agents
- Fix useragent — context recreation preserves cookies/storage/URLs
- wait --networkidle / --load / --domcontentloaded flags
- console --errors filter (error + warning only)
- cookie-import <json-file> with auto-fill domain from page URL
- 166 integration tests (was ~63)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

browse/test/test-server.ts

@@ -14,6 +14,16 @@ export function startTestServer(port: number = 0): { server: ReturnType<typeof B
     hostname: '127.0.0.1',
     fetch(req) {
       const url = new URL(req.url);
+
+      // Echo endpoint — returns request headers as JSON
+      if (url.pathname === '/echo') {
+        const headers: Record<string, string> = {};
+        req.headers.forEach((value, key) => { headers[key] = value; });
+        return new Response(JSON.stringify(headers, null, 2), {
+          headers: { 'Content-Type': 'application/json' },
+        });
+      }
+
       let filePath = url.pathname === '/' ? '/basic.html' : url.pathname;
 
       // Remove leading slash