feat: add draw.rounded_rectangle primitive

[wkentaro]

Summary

• Add imgviz.draw.rounded_rectangle(image, yx1, yx2, radius, fill=None, outline=None, width=0) and its in-place pair rounded_rectangle_, for arc-cornered rectangles.
• Thin wrapper over PIL's native ImageDraw.rounded_rectangle, mirroring the existing draw.rectangle / rectangle_ pair exactly: same yx1/yx2 (min/max vertex) convention with the yx→xy swap, same Ink / get_pil_ink / require_fill_or_outline handling, same width=0 default (kept for sibling consistency rather than diverging a single primitive).
• PIL's per-corner corners kwarg is intentionally not exposed — the issue spec is (yx1, yx2, radius) and no caller needs selective rounding yet; it can be added when one does.

Test plan

• tests added: tests/unit/draw/_rounded_rectangle_test.py (rendered outline, the fill/outline guard, corner-clipping pixel check, in-place mutation)
• uv run pytest tests/unit/draw/ — 33 passed
• uv run ruff check / uv run ruff format --check / uv run ty check clean on touched files

Closes #154

[wkentaro]

Added examples/rounded_rectangle.py (and refreshed the README table) so the rendering is reviewable. It draws three boxes with radius=0 (sharp), radius=40 (rounded), and radius=100 (pill) on lena:

Regenerate with uv run python examples/rounded_rectangle.py --save.