fix(palette): fix u16 overflow in luma() causing grayscale theme crash

[wdw8276]

Summary

• luma() in palette.rs uses u16 for intermediate arithmetic
• u16::from(r) * 299 overflows u16::MAX (65535) when r >= 220
• u16::from(g) * 587 overflows when g >= 112
• In debug builds, this causes a panic (attempt to add with overflow) whenever the grayscale theme processes a bright RGB color
• Fix: use u32 for intermediate calculations (result still fits in u8)

Reproduction

Switch to the Grayscale theme (/theme → select option 4) in a debug build — TUI immediately crashes with a panic logged to ~/.deepseek/crashes/.

Fix

// Before
fn luma(r: u8, g: u8, b: u8) -> u8 {
    (((u16::from(r) * 299) + (u16::from(g) * 587) + (u16::from(b) * 114)) / 1000) as u8
}

// After
fn luma(r: u8, g: u8, b: u8) -> u8 {
    (((u32::from(r) * 299) + (u32::from(g) * 587) + (u32::from(b) * 114)) / 1000) as u8
}

[gemini-code-assist]

Code Review

This pull request updates the luma function in crates/tui/src/palette.rs to use u32 for intermediate calculations, effectively preventing integer overflows. The reviewer suggested improving the accuracy of the luminance mapping by adding a rounding offset before division and recommended simplifying the expression by removing redundant parentheses.

[gemini-code-assist]

While the switch to u32 correctly prevents the overflow and subsequent panic, the current implementation uses integer division which truncates the result. For more accurate luminance mapping to the grayscale palette, consider adding a rounding offset (500) before dividing by 1000. Additionally, the inner parentheses around the multiplications are redundant and can be removed to improve readability.

Suggested change

	(((u32::from(r) * 299) + (u32::from(g) * 587) + (u32::from(b) * 114)) / 1000) as u8
	((u32::from(r) * 299 + u32::from(g) * 587 + u32::from(b) * 114 + 500) / 1000) as u8

[wdw8276]

Thanks for the review! Both suggestions have been applied in the latest commit:

• Added +500 before /1000 for proper integer rounding
• Removed the redundant inner parentheses

[Hmbown]

Thanks for the grayscale overflow patch. This was harvested into v0.8.40 release PR #1823 as commit ae1bb9d, and #1823 is now CI-green. Closing as superseded by the release branch; thank you for catching the crash path.