Fix IME key events blocked by ctrl fast path and missing layout change detection

The ctrl fast path unconditionally returned after calling ghostty_surface_key,
even when it returned false (e.g. ignore keybindings), preventing IMEs from
receiving Ctrl-modified key events. Now falls through to interpretKeyEvents
when the key is not handled.

Also adds keyboard layout change detection around interpretKeyEvents (matching
Ghostty upstream) so that IME-triggered layout switches cause an early return
instead of sending the key to Ghostty.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Sources/KeyboardLayout.swift

@@ -0,0 +1,14 @@
+import Carbon
+
+class KeyboardLayout {
+    /// Return a string ID of the current keyboard input source.
+    static var id: String? {
+        if let source = TISCopyCurrentKeyboardInputSource()?.takeRetainedValue(),
+           let sourceIdPointer = TISGetInputSourceProperty(source, kTISPropertyInputSourceID) {
+            let sourceId = unsafeBitCast(sourceIdPointer, to: CFString.self)
+            return sourceId as String
+        }
+
+        return nil
+    }
+}