refactor(draw): extract require_pil_image guard into _ink

[wkentaro]

Closes #192.

The in-place draw primitives line_ and polygon_ each repeated the same
three-line PIL image type guard. This extracts it into require_pil_image in
imgviz/draw/_ink.py, alongside the existing require_fill_or_outline, and
replaces both inline copies. Behavior and the TypeError message are unchanged.

Summary

• Add require_pil_image(image) to imgviz/draw/_ink.py.
• Replace the inline guard in line_ and polygon_ with a call to it.

Scope note: only line_ and polygon_ carry the inline guard on main today
(arrow_ arrives with #193 and can adopt the helper there). The other in-place
primitives (circle_, ellipse_, rectangle_, etc.) never had a guard;
applying one to them is a behavior change (they would start raising TypeError
instead of a Pillow AttributeError), so that uniform rollout is tracked
separately rather than folded into this dedupe.

Test plan

• full suite uv run --extra all pytest -n=auto tests (251 passed)
• the existing polygon_ "rejects non-PIL image" test still passes, exercising the extracted helper
• uv run ruff check, ty check on the touched files