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-lib/tests/surface.rs

@@ -519,6 +519,28 @@ fn selection_and_clipboard_round_trip() {
 /// A child that prints the hex of the first two bytes it receives raw.
 const HEX_TWO: &str = "stty raw -echo; dd bs=1 count=2 2>/dev/null | od -An -tx1 | tr -d ' \\n'; printf '\\nHEX-DONE\\n'; sleep 5";
 
+#[test]
+fn key_text_and_preedit_are_bytes_not_lossy_utf8() {
+    // Key text is a byte slice (Zig `[]const u8`): non-UTF-8 text reaches
+    // the pty unchanged.
+    let h = host!("");
+    let s = h.surface(HEX_TWO);
+    let raw = CString::new(&b"\xff\xfe"[..]).unwrap();
+    assert!(unsafe { ghostty_surface_key(s, key(1, KEY_A, 0, Some(&raw))) });
+    h.wait_screen(s, "fffe");
+    unsafe { ghostty_surface_free(s) };
+
+    // Invalid preedit bytes are rejected like Zig (logged error, no panic).
+    let s = h.surface("printf 'READY\\n'; sleep 30");
+    h.wait_screen(s, "READY");
+    let p = b"a\xffb";
+    unsafe {
+        ghostty_surface_preedit(s, p.as_ptr().cast(), p.len());
+        ghostty_surface_preedit(s, ptr::null(), 0);
+        ghostty_surface_free(s);
+    }
+}
+
 #[test]
 fn clipboard_completion_and_binding_action_keep_raw_bytes() {
     // Clipboard contents are bytes: 0xFF must reach the child as 0xFF.