terminal: Use conventional XTerm indexed color values (#32200)

Fixes rendering of colors in the terminal to use XTerm's idiosyncratic standard steps instead of the range that was previously in use. Matches the behavior of Alacritty, Ghostty, iTerm2, and every other terminal emulator I've looked at.

Release Notes:

- Fixed rendering of terminal colors for the XTerm 256-color indexed color palette.

crates/terminal/src/terminal.rs

@@ -2102,17 +2102,21 @@ pub fn get_color_at_index(index: usize, theme: &Theme) -> Hsla {
         13 => colors.terminal_ansi_bright_magenta,
         14 => colors.terminal_ansi_bright_cyan,
         15 => colors.terminal_ansi_bright_white,
-        // 16-231 are mapped to their RGB colors on a 0-5 range per channel
+        // 16-231 are a 6x6x6 RGB color cube, mapped to 0-255 using steps defined by XTerm.
+        // See: https://github.com/xterm-x11/xterm-snapshots/blob/master/256colres.pl
         16..=231 => {
-            let (r, g, b) = rgb_for_index(index as u8); // Split the index into its ANSI-RGB components
-            let step = (u8::MAX as f32 / 5.).floor() as u8; // Split the RGB range into 5 chunks, with floor so no overflow
-            rgba_color(r * step, g * step, b * step) // Map the ANSI-RGB components to an RGB color
+            let (r, g, b) = rgb_for_index(index as u8);
+            rgba_color(
+                if r == 0 { 0 } else { r * 40 + 55 },
+                if g == 0 { 0 } else { g * 40 + 55 },
+                if b == 0 { 0 } else { b * 40 + 55 },
+            )
         }
-        // 232-255 are a 24 step grayscale from black to white
+        // 232-255 are a 24-step grayscale ramp from (8, 8, 8) to (238, 238, 238).
         232..=255 => {
             let i = index as u8 - 232; // Align index to 0..24
-            let step = (u8::MAX as f32 / 24.).floor() as u8; // Split the RGB grayscale values into 24 chunks
-            rgba_color(i * step, i * step, i * step) // Map the ANSI-grayscale components to the RGB-grayscale
+            let value = i * 10 + 8;
+            rgba_color(value, value, value)
         }
         // For compatibility with the alacritty::Colors interface
         // See: https://github.com/alacritty/alacritty/blob/master/alacritty_terminal/src/term/color.rs