macos: make QuickTerminalSize config C ABI compatible (#9837)

Hello! I did read `CONTRIBUTING.md`. I understand that the preference is
to receive pull requests for existing open issues and acknowledge that
I'm skipping a step wrt the preferred workflow since there aren't any
open issues yet for #8419. Living dangerously and hoping for the best
here. I hope you'll see that I'm acting in good faith 🤞.

In my attempts to debug why `quick-terminal-size` config wasn't working
reliably for me, I discovered that the
[`ghostty_config_quick_terminal_size_s`](08c9661683/include/ghostty.h (L460-L480))
struct used to initialize `QuickTerminalSize` in Swift-land, more often
than not, didn't match what I had in my config file (e.g.
`quick-terminal-size = 75%, 50%`)

<details>


08c9661683/include/ghostty.h (L460-L480)


08c9661683/macos/Sources/Ghostty/Ghostty.Config.swift (L507-L510)

</details>

Almost all mismatches seemed to be downstream of `tag` (for `primary`
and/or `secondary` `Size`) being an unexpected value for the
`ghostty_quick_terminal_size_tag_e` enum type, which led runtime
execution to fall into the `default` branch in this `switch` control
flow.


08c9661683/macos/Sources/Features/QuickTerminal/QuickTerminalSize.swift (L27-L38)

Looking at `src/config/CApi.zig`, `src/config/c_get.zig`,
`src/config/Config.zig`, and [Zig documentation for `extern
enum`](https://ziglang.org/documentation/master/#extern-enum), led me to
conclude that the crux of the issue is lack of guaranteed C ABI
compatibility for `Tag`


08c9661683/src/config/Config.zig (L7986-L7990)


08c9661683/include/ghostty.h (L461-L465)

Further research revealed that based on C language spec alone, one
cannot assume a fixed width for `enum` and that the behaviour is
compiler dependant. But given how the Zig documentation suggests using
`enum(c_int)` for C-ABI-compatible enums and extern enums + comments
elsewhere in `Config.zig` suggesting that `enum(c_int)` was chosen for
extern compatibility,


08c9661683/src/config/Config.zig (L4877)

I think that this fix (changing `enum(u8)` to `enum(c_int)`) is likely
the most straightforward and appropriate one.

macos/Sources/Features/QuickTerminal/QuickTerminalSize.swift

@@ -33,6 +33,7 @@ struct QuickTerminalSize {
             case GHOSTTY_QUICK_TERMINAL_SIZE_PIXELS:
                 self = .pixels(cStruct.value.pixels)
             default:
+                assertionFailure()
                 return nil
             }
         }

src/config/Config.zig

@@ -7987,7 +7987,8 @@ pub const QuickTerminalSize = struct {
             tag: Tag,
             value: Value,
 
-            pub const Tag = enum(u8) { none, percentage, pixels };
+            /// c_int because it needs to be extern compatible
+            pub const Tag = enum(c_int) { none, percentage, pixels };
 
             pub const Value = extern union {
                 percentage: f32,