snippets: add gpu-draw-handler.py (viewport overlay with a removable draw handler) #500

What

Add a snippet snippets/gpu-draw-handler.py: a viewport overlay that registers a SpaceView3D.draw_handler_add callback, keeps the handle, and removes it in unregister(). Draw one POLYLINE_UNIFORM_COLOR line with gpu.state.blend_set('ALPHA'), then restore 'NONE'.

Why

The gpu-drawing-and-overlays skill teaches this pattern, but there is no copy-paste snippet, and snippets are what people grab first.

How

• Copy the "overlay pattern" block from skills/gpu-drawing-and-overlays/SKILL.md. Keep the snippet between 5 and 75 lines (tests/check_counts.py enforces the range).
• Update the snippet count from 29 to 30 wherever python tests/check_counts.py reports it.
• Run python tests/run_all.py, then open a PR.

No Blender install needed: snippets are syntax-checked in CI.