feat: demo enrichment pipeline for GroundingTarget data (#261)

Add two scripts for populating GroundingTarget data on demo click steps:

- enrich_demo_targets.py: Enriches each click step with GroundingTarget
  metadata (target_type, crop_bbox, click_offset, nearby_text) using OCR
  when real screenshots are available, or description-derived heuristics
  when they are not. Idempotent and works offline.

- record_demo_screenshots.py: Replays a demo on a live WAA VM, capturing
  before/after screenshots at each step, then updates the demo JSON with
  real screenshot paths for subsequent enrichment.

Both scripts use fire for CLI, handle the existing demo JSON format, and
integrate with grounding.py GroundingTarget.to_dict()/from_dict().

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: abrichr