Automatically adjust ANSI color contrast (#34033)

Closes #33253 in a way that doesn't regress #32175 - namely,
automatically adjusts the contrast between the foreground and background
text in the terminal such that it's above a certain threshold. The
threshold is configurable in settings, and can be set to 0 to turn off
this feature and use exactly the colors the theme specifies even if they
are illegible.

## One Light Theme Before
<img width="220" alt="Screenshot 2025-07-07 at 6 00 47 PM"
src="https://github.com/user-attachments/assets/096754a6-f79f-4fea-a86e-cb7b8ff45d60"
/>

(Last row is highlighted because otherwise the text is unreadable; the
foreground and background are the same color.)

## One Light Theme After

(This is with the new default contrast adjustment setting.)

<img width="215" alt="Screenshot 2025-07-07 at 6 22 02 PM"
src="https://github.com/user-attachments/assets/b082fefe-76f5-4231-b704-ff387983a3cb"
/>

This approach was inspired by @mitchellh's use of automatic contrast
adjustment in [Ghostty](https://ghostty.org/) - thanks, Mitchell! The
main difference is that we're using APCA's formula instead of WCAG for
[these
reasons](https://khan-tw.medium.com/wcag2-are-you-still-using-it-ui-contrast-visibility-standard-readability-contrast-f34eb73e89ee).

Release Notes:

- Added automatic dynamic contrast adjustment for terminal foreground
and background colors

crates/terminal/src/terminal_settings.rs

@@ -49,6 +49,7 @@ pub struct TerminalSettings {
     pub max_scroll_history_lines: Option<usize>,
     pub toolbar: Toolbar,
     pub scrollbar: ScrollbarSettings,
+    pub minimum_contrast: f32,
 }
 
 #[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
@@ -229,6 +230,21 @@ pub struct TerminalSettingsContent {
     pub toolbar: Option<ToolbarContent>,
     /// Scrollbar-related settings
     pub scrollbar: Option<ScrollbarSettingsContent>,
+    /// The minimum APCA perceptual contrast between foreground and background colors.
+    ///
+    /// APCA (Accessible Perceptual Contrast Algorithm) is more accurate than WCAG 2.x,
+    /// especially for dark mode. Values range from 0 to 106.
+    ///
+    /// Based on APCA Readability Criterion (ARC) Bronze Simple Mode:
+    /// https://readtech.org/ARC/tests/bronze-simple-mode/
+    /// - 0: No contrast adjustment
+    /// - 45: Minimum for large fluent text (36px+)
+    /// - 60: Minimum for other content text
+    /// - 75: Minimum for body text
+    /// - 90: Preferred for body text
+    ///
+    /// Default: 0 (no adjustment)
+    pub minimum_contrast: Option<f32>,
 }
 
 impl settings::Settings for TerminalSettings {
@@ -237,7 +253,18 @@ impl settings::Settings for TerminalSettings {
     type FileContent = TerminalSettingsContent;
 
     fn load(sources: SettingsSources<Self::FileContent>, _: &mut App) -> anyhow::Result<Self> {
-        sources.json_merge()
+        let settings: Self = sources.json_merge()?;
+
+        // Validate minimum_contrast for APCA
+        if settings.minimum_contrast < 0.0 || settings.minimum_contrast > 106.0 {
+            anyhow::bail!(
+                "terminal.minimum_contrast must be between 0 and 106, but got {}. \
+                APCA values: 0 = no adjustment, 75 = recommended for body text, 106 = maximum contrast.",
+                settings.minimum_contrast
+            );
+        }
+
+        Ok(settings)
     }
 
     fn import_from_vscode(vscode: &settings::VsCodeSettings, current: &mut Self::FileContent) {