[davidbrochart]

I used Codex for this PR, here's what it addressed:

Fractional display scaling leaves visible seams between adjacent terminal cell backgrounds. For example, at 125% scaling a 9 CSS-pixel cell is 11.25 device pixels wide; drawing each background independently antialiases shared edges against the underlying theme color. This appears as vertical stripes across solid backgrounds.

Snap cell background and row-clear bounds to shared physical-pixel edges, including selected, inverse, and wide cells. Round canvas backing dimensions and resize comparisons consistently so fractional sizes do not trigger repeated canvas resets. Glyph positioning is unchanged.

Validation:

• Added renderer regression coverage at device-pixel ratios 1, 1.1, 1.25, 1.5, and 2, including shared horizontal/vertical edges and rounded canvas dimensions.
• A separate real-canvas pixel check reproduced mixed background colors before the fix and verified a uniform RGB background at all five scales afterward.
• Formatting, lint, and TypeScript checks pass.
• Full test suite: 379 passed, 0 failed.
• Full WASM and JavaScript build passes (Zig 0.15.2).