Type: input method composition (Japanese IME) in the Type tool

Typing with an input method (the macOS Japanese IME and the like) used to
insert the raw keys (romaji came in as ASCII): egui's Ime events weren't
handled.

- Tools gain `ime_preedit` / `composing` / `ime_caret`; the Type tool lays
  the marked text out in place inside its typing session, underlined (the
  clause being converted thicker), and commits it through `text_input`.
  Character ranges from the IME convert to byte ranges in one place. A
  cancelled composition leaves no empty undo step; an empty clear with
  nothing marked never deletes the selection.
- The UI sends Text and Ime events to the tool in order, sends IMEOutput
  every frame while text is edited (what lets egui-winit enable the IME
  and place its candidate window at the caret, or the converting clause),
  and holds back keys, Enter/Escape, clipboard, single-key shortcuts and
  menu commands (Undo, Cut, Select All… also from the native menu) while
  composing. A click, a tool switch or a style change ends the
  composition with the marked text kept as typed; on macOS the IME is
  told to discard it (NSTextInputContext.discardMarkedText) so it doesn't
  come back in the next input.
- Undo during a typing session also ends the Type tool's session, so its
  caret and marked text follow the document.
- Tests: tools (char → byte ranges, underlines and candidate position),
  engine (compose → commit is one step, cancel leaves no step, a click
  commits, Undo mid-composition is safe), ui-egui (synthetic Ime events:
  compose, commit, ⌘Z held back, IME output, click and app switch).

Checked by hand on macOS 27 with the Japanese IME (romaji input, convert,
commit, cancel, Undo/Redo after commit, click away, Latin mode, app
switch). Not checked: Windows and Linux IMEs, the web build.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/vectorcraft/src/main.rs

@@ -42,12 +42,27 @@ impl eframe::App for App {
     }
     fn ui(&mut self, ui: &mut egui::Ui, _frame: &mut eframe::Frame) {
         self.0.ui(ui);
+        #[cfg(target_os = "macos")]
+        if self.0.take_ime_discard() {
+            discard_marked_text();
+        }
     }
     fn on_exit(&mut self) {
         save_prefs(&self.0);
     }
 }
 
+/// Tell the macOS input method to drop its composition (the Type tool kept the marked text as
+/// typed). winit's IME toggle only clears its own copy, so the IME would type it again.
+#[cfg(target_os = "macos")]
+fn discard_marked_text() {
+    if let Some(mtm) = objc2::MainThreadMarker::new()
+        && let Some(ic) = objc2_app_kit::NSTextInputContext::currentInputContext(mtm)
+    {
+        ic.discardMarkedText();
+    }
+}
+
 /// Where UI preferences live: ~/Library/Application Support/VectorCraft (macOS),
 /// %APPDATA%\VectorCraft (Windows), $XDG_CONFIG_HOME or ~/.config/vectorcraft (Linux).
 fn prefs_path() -> Option<std::path::PathBuf> {