feat: add tint for whole-image color wash

[wkentaro]

Summary

• Add imgviz.tint(image, color, alpha=0.3) for a whole-image color wash, e.g. flagging a thumbnail in a tile sheet by red-tinting it: out = (1-alpha)*image + alpha*solid_color.
• color accepts any cmap-compatible value (name, hex, or (r, g, b) tuple; ints in [0, 255] or floats in [0, 1]), parsed via cmap.Color.
• alpha=0 returns the input unchanged, alpha=1 the solid color; out-of-[0, 1] alpha raises. Supports uint8 [0, 255] and float [0, 1] with @overload dtype preservation. RGB-only, with an error pointing to imgviz.asrgb.

Test plan

• tests added: tests/unit/_tint_test.py — wash changes pixels, alpha=0 returns an unchanged copy, alpha=1 is the solid color, color formats (name/hex/tuple), float dtype preserved, and guards for non-RGB shape / invalid dtype / invalid alpha (negative, >1, NaN)
• example added: uv run python examples/tint.py --save; README table refreshed via generate_readme.py
• uv run pytest tests/unit/_tint_test.py — 13 passed
• uv run ruff check / uv run ruff format --check / uv run ty check clean on touched files (incl. test + example)

Closes #130