fix-deviant(gtk): reject malformed +new-window arguments instead of aborting

Intentional deviation from Ghostty 1.3.1 (DEVIANT_FIXES.md row 31).
Zig's apprt/gtk/ipc/new_window.zig:49-53 builds the D-Bus argument
array from raw bytes as a *trusted* GVariant; an argument with invalid
UTF-8 or an interior NUL violates GVariant's string invariants, which
GLib turns into a process abort. The Rust port mirrored this with an
explicit abort().

new_window now validates every argument before touching the bus: a bad
argument prints an error to stderr and returns Error::IpcFailed, so the
CLI exits with failure and nothing crashes. Valid arguments are sent
byte-identically to Ghostty.

Test Plan:
- cargo test -p ghostty-apprt-gtk new_window
  (invalid_dbus_strings_are_rejected_without_abort)

docs/rewrite/DEVIANT_FIXES.md

@@ -56,6 +56,7 @@ when this file was written.
 | 28 | CLI: `+list-colors` plain output | `cli/list_colors.zig:59-72` | 4096-byte buffer, never flushed; piped output stops midway through `PeachPuff3` (12,297 bytes). | Flushed (15,434 bytes). | 6286e02 | none; found by RV11 (`review-rv11-codex.md`) |
 | 29 | CLI: `+list-themes` diagnostics | `cli/list_themes.zig:124-131,171` | The messages ("Could not find the Ghostty resources directory", "No themes found...") go to a 4096-byte stderr buffer that is never flushed, so they are lost. | Flushed when the action ends. | `410d8fc` fix-deviant(cli) | `zig_writer::tests::unflushed_buffer_is_flushed_on_drop` |
 | 30 | CLI: stdout as a regular file | `std.fs.File.writer` (Zig 0.15.2) as used in `cli/show_face.zig:68-73`, `cli/help.zig:34`, `cli/validate_config.zig:43-46`, `cli/show_config.zig:84` | The writer `pwrite`s at its own offset starting at 0: with stdout redirected to a file, output overwrites itself and earlier file content, and `Writer.end` truncates. | Writes sequentially at the inherited descriptor's current offset (no `pwrite` from offset 0, so earlier content and the writers' own output are not overwritten; this appends only when the descriptor is opened for append, e.g. `>>`); no truncation. | `410d8fc` fix-deviant(cli) (append behaviour first in `f76179f`) | `zig_writer::tests::regular_file_output_is_complete_and_appended` |
+| 31 | GTK: `+new-window` D-Bus arguments | `apprt/gtk/ipc/new_window.zig:49-53` (`Bytes.new(ptr, len + 1)` + `Variant.newFromBytes(..., trusted = true)`) | Arguments are passed as trusted GVariant strings without validation; invalid UTF-8 or an interior NUL makes GIO abort the process while serializing (input-triggered crash). | `arguments_variant` validates first and returns `Error::IpcFailed` (message on stderr, no D-Bus call); valid arguments are byte-identical to Zig. | uncommitted (Z18) | `ipc::new_window::tests::invalid_dbus_strings_are_rejected_without_abort` (`ghostty-apprt-gtk`) |
 
 ## Upstream quirks deliberately kept (not fixed)