fix(render): remove seams between color blocks

[cpendery (cpendery)]

Summary

• rasterize consecutive same-color backgrounds as runs whose shared cell edges snap to identical output pixels
• align block, underline, and bar cursors to that same pixel grid so fractional zoom cannot leave a gap beside the cursor
• use explicit fill_antialiased_rect and fill_pixel_rect helpers instead of the ambiguous regular/crisp rectangle distinction
• cover multi-row color runs, a full-width trailing run, and cursor adjacency at gap-prone fractional zoom levels
• run the static-media generator, retain its visually changed 25% APNG, and add fractional pixel-alignment GIF/PNG render snapshots

Validation

• cargo clippy -p tui-test-rs --all-targets --all-features -- -D warnings
• cargo test -p tui-test-rs --all-features -- --test-threads=1
• bash scripts/regenerate-static-media.sh
• cargo test -p tui-test-rs --all-features single_frame_gif_and_png_renders_match_snapshots

Before and after

Both captures use the same terminal content and --zoom 0.625. The first two rows are equal-width 30-cell comparisons with the cursor hidden. The third row deliberately shows the cursor after a 29-cell green run, isolating cursor alignment instead of accidentally appending it to the color comparison.

The solid bar drops from 30 off-color seam pixels to 0. At the cursor boundary, the before image contains a blended transition pixel; the after image ends the green run and starts the full cursor cell on one shared pixel edge.

Before	After