fix(input): carry key and preedit text as bytes like Zig

Zig's KeyEvent.utf8 and the embedded API's key/preedit text are byte
slices passed through to key encoding and the pty; the port converted
them lossily to UTF-8.
- KeyEvent.utf8 is &[u8] end to end (ghostty-input encoder, core,
  ghostty-lib C API, libghostty-vt key events, GTK callers). Where Zig
  validates with Utf8View (alternates, modify-other-keys, ctrl CSI-u,
  associated text, binding unicode lookup) invalid text takes the same
  "no UTF-8" fallback.
- Invalid preedit UTF-8 behaves like Zig's `try Utf8View.init`: the
  prior preedit is cleared and marked dirty, the callback returns an
  error (logged by the C API) and no preedit is set.
- The inspector key widget shows invalid key text lossily, display only.

Test Plan:
- cargo test --workspace --no-fail-fast (3407 passed)
- cargo test -p ghostty-lib --features test-platform (C-API key text
  0xff 0xfe reaches the pty unchanged)
- just check-macos (both), just check-windows, just check-wasm

crates/ghostty-input/src/binding/set.rs

@@ -574,8 +574,9 @@ impl Set {
 
         // If our UTF-8 text is exactly one codepoint, we try to match that.
         if !event.utf8.is_empty() {
-            let mut it = event.utf8.chars();
-            // No codepoints or multiple codepoints drops to invalid format
+            // Invalid UTF-8, no codepoints or multiple codepoints drops to
+            // invalid format.
+            let mut it = crate::key_encode::utf8_chars(event.utf8).unwrap_or_else(|| "".chars());
             if let (Some(cp), None) = (it.next(), it.next()) {
                 trigger.key = TriggerKey::Unicode(cp as u32);
                 if let Some(v) = self.get_entry(trigger) {

crates/ghostty-input/src/binding/tests.rs

@@ -1097,7 +1097,7 @@ fn ev(key: Key, mods: Mods, utf8: &str, unshifted: u32) -> KeyEvent<'_> {
     KeyEvent {
         key,
         mods,
-        utf8,
+        utf8: utf8.as_bytes(),
         unshifted_codepoint: unshifted,
         ..KeyEvent::default()
     }