feat: add draw.arrow primitive

[wkentaro]

Summary

• Add imgviz.draw.arrow(image, yx1, yx2, fill, width=1, head_length_ratio=0.1, head_angle=30.0) and its in-place pair arrow_, drawing a shaft from yx1 (tail) to yx2 (tip) with an arrowhead at the tip.
• The arrowhead is an open V of two barbs computed by rotating the shaft unit vector by ±head_angle, so the geometry stays correct for any direction (the "not trivially composable" justification per ADR 0002).
• head_length_ratio sizes the head as a fraction of the shaft length (cv2 arrowedLine's tipLength convention), so it stays sensible at any image scale; head_angle is the head half-angle in degrees.
• Single fill stroke color like draw.line; yx1/yx2 accept tuples/lists/arrays (ArrayLike) and are shape-validated, matching draw.line.

Test plan

• tests added: tests/unit/draw/_arrow_test.py — happy path, head-adds-over-plain-line, head-at-tip for horizontal and diagonal arrows, head_length_ratio scaling, zero-length skips head, in-place mutation, non-PIL and bad-shape guards
• uv run pytest tests/unit/draw/ — 47 passed
• uv run ruff check / uv run ruff format --check / uv run ty check clean on touched files

Closes #152

[wkentaro]

lint is failing