feat: add draw.progress_bar overlay primitive

[wkentaro]

Summary

• Add imgviz.draw.progress_bar(image, yx1, yx2, value, fill, background, outline=None) and its in-place pair progress_bar_, for inline horizontal progress bars (confidence indicators, quality scores, tile-sheet strips).
• value is clamped to [0, 1]: 0 draws the track only, 1 fills it fully. Composes draw.rectangle_ (track + filled portion + optional outline); the fill rect is skipped at value 0 to avoid a zero-width rectangle.
• Named outline (not border) and requires fill/background with no defaults, matching the rest of the draw module.

Test plan

• tests added: tests/unit/draw/_progress_bar_test.py — value 0 (track only), value 1 (fully filled), half-fill pixel check, clamping (1.5 and -1.0), outline drawn at every fill level, outline does not overpaint the interior, and in-place mutation
• example added: uv run python examples/progress_bar.py --save; README table refreshed via generate_readme.py
• uv run pytest tests/unit/draw/_progress_bar_test.py — 11 passed
• uv run ruff check / uv run ruff format --check / uv run ty check clean on touched files (incl. test + example)

Closes #132