macos: implement audio bell support with bell-audio-path (#11154)

## Summary

This extends the macOS bell implementation to support the `audio` bell
feature, bringing it to parity with GTK/Linux.

Previously, macOS only had the `system` feature (`NSSound.beep()`). This
PR adds:

- **`audio` bell feature on macOS**: plays the file at `bell-audio-path`
using `NSSound(contentsOfFile:)`, respecting `bell-audio-volume`
- **`cval()` on the `Path` type**: allows `Path` values (a union type)
to be returned through the C API, which is needed for Swift to read
`bell-audio-path`
- **Removes `(GTK only)` restriction** from `bell-audio-path` and
`bell-audio-volume` documentation

## How it works

In `AppDelegate.swift`, when the bell rings and the `audio` feature is
enabled, Ghostty now:
1. Reads `bell-audio-path` from config
2. Loads it as an `NSSound`
3. Applies `bell-audio-volume` and plays it

Falls back gracefully if the path is not set or the file cannot be
loaded.

## Example config

```
bell-features = audio
bell-audio-path = /System/Library/Sounds/Glass.aiff
bell-audio-volume = 0.8
```

## Testing

- Set `bell-features = audio` and `bell-audio-path` to any valid audio
file
- Trigger a bell with `echo -e '\a'`
- Audio should play at the configured volume

include/ghostty.h

@@ -463,6 +463,12 @@ typedef struct {
 
 // Config types
 
+// config.Path
+typedef struct {
+  const char* path;
+  bool optional;
+} ghostty_config_path_s;
+
 // config.Color
 typedef struct {
   uint8_t r;

macos/Sources/App/macOS/AppDelegate.swift

@@ -777,6 +777,14 @@ class AppDelegate: NSObject,
             NSSound.beep()
         }
 
+        if ghostty.config.bellFeatures.contains(.audio) {
+            if let configPath = ghostty.config.bellAudioPath,
+               let sound = NSSound(contentsOfFile: configPath.path, byReference: false) {
+                sound.volume = ghostty.config.bellAudioVolume
+                sound.play()
+            }
+        }
+
         if ghostty.config.bellFeatures.contains(.attention) {
             // Bounce the dock icon if we're not focused.
             NSApp.requestUserAttention(.informationalRequest)

macos/Sources/Ghostty/Ghostty.Config.swift

@@ -134,6 +134,23 @@ extension Ghostty {
             return .init(rawValue: v)
         }
 
+        var bellAudioPath: ConfigPath? {
+            guard let config = self.config else { return nil }
+            var v = ghostty_config_path_s()
+            let key = "bell-audio-path"
+            guard ghostty_config_get(config, &v, key, UInt(key.lengthOfBytes(using: .utf8))) else { return nil }
+            let path = String(cString: v.path)
+            return path.isEmpty ? nil : ConfigPath(path: path, optional: v.optional)
+        }
+
+        var bellAudioVolume: Float {
+            guard let config = self.config else { return 0.5 }
+            var v: Double = 0.5
+            let key = "bell-audio-volume"
+            _ = ghostty_config_get(config, &v, key, UInt(key.lengthOfBytes(using: .utf8)))
+            return Float(v)
+        }
+
         var notifyOnCommandFinish: NotifyOnCommandFinish {
             guard let config = self.config else { return .never }
             var v: UnsafePointer<Int8>?

macos/Sources/Ghostty/Ghostty.ConfigTypes.swift

@@ -2,6 +2,12 @@
 // can get typed information without depending on all the dependencies of GhosttyKit.
 
 extension Ghostty {
+    /// A configuration path value that may be optional or required.
+    struct ConfigPath: Sendable {
+        let path: String
+        let optional: Bool
+    }
+
     /// macos-icon
     enum MacOSIcon: String, Sendable {
         case official

src/config/Config.zig

@@ -3087,7 +3087,7 @@ keybind: Keybinds = .{},
 /// the path is not absolute, it is considered relative to the directory of the
 /// configuration file that it is referenced from, or from the current working
 /// directory if this is used as a CLI flag. The path may be prefixed with `~/`
-/// to reference the user's home directory. (GTK only)
+/// to reference the user's home directory.
 ///
 /// Available since: 1.2.0
 @"bell-audio-path": ?Path = null,
@@ -3095,7 +3095,6 @@ keybind: Keybinds = .{},
 /// If `audio` is an enabled bell feature, this is the volume to play the audio
 /// file at (relative to the system volume). This is a floating point number
 /// ranging from 0.0 (silence) to 1.0 (as loud as possible). The default is 0.5.
-/// (GTK only)
 ///
 /// Available since: 1.2.0
 @"bell-audio-volume": f64 = 0.5,

src/config/path.zig

@@ -32,6 +32,20 @@ pub const Path = union(enum) {
         return std.meta.eql(self, other);
     }
 
+    /// ghostty_config_path_s
+    pub const C = extern struct {
+        path: [*:0]const u8,
+        optional: bool,
+    };
+
+    /// Returns the path as a C-compatible struct.
+    pub fn cval(self: Path) C {
+        return switch (self) {
+            .optional => |path| .{ .path = path.ptr, .optional = true },
+            .required => |path| .{ .path = path.ptr, .optional = false },
+        };
+    }
+
     /// Parse the input and return a Path. A leading `?` indicates that the path
     /// is _optional_ and an error should not be logged or displayed to the user
     /// if that path does not exist. Otherwise the path is required and an error