feat(cdp): signal when the ref list is truncated in look()

[nipunrajk]

What

• Tracks truncatedCount in compactAxTree by continuing the loop to count matching elements after the MAX_REFS limit is reached.
• Appends a hint to the end of the look() text output when truncated: \n… N more elements below — scroll or filter.
• Adds assertions to verify truncatedCount in snapshot.test.ts.

Why

Fixes #2. Prevents the LLM agent from assuming it has captured the entire page when the element list is truncated, letting it know it needs to scroll or filter.

Checklist

• pnpm test and pnpm -r typecheck pass
• Docs/README updated if behavior changed