Fix Palette.C array size typo: [265] → [256]

The C struct Palette.C declared colors as [265]Color.C, but the
terminal palette is 256 colors (terminal.color.Palette = [256]RGB)
and the C header ghostty_config_palette_s correctly uses colors[256].

The mismatch causes ghostty_config_get to write 265×3 = 795 bytes
through a pointer sized for 256×3 = 768 bytes, producing a 27-byte
buffer overflow. On macOS Release builds with stack protector enabled,
this triggers __stack_chk_fail → SIGABRT on launch.

src/config/Config.zig

@@ -5645,7 +5645,7 @@ pub const Palette = struct {
 
     /// ghostty_config_palette_s
     pub const C = extern struct {
-        colors: [265]Color.C,
+        colors: [256]Color.C,
     };
 
     pub fn cval(self: Self) Palette.C {