W33: Mode-change flash in status bar

When the editor mode changes (e.g. Esc → Normal, i → Insert),
the mode tag in the status bar flashes to the accent colour
for ~300ms. Provides instant visual feedback for mode changes.

- Editor::mode_flash: Option<Instant> field
- Editor::trigger_mode_flash() / mode_flash_progress()
- Editor::set_mode() helper that triggers the flash on actual
  mode changes (no-op when setting the same mode)
- All self.mode = Mode::* sites in handlers.rs updated to use
  set_mode() so the flash triggers consistently
- status_spans uses accent + cursor_fg for the mode tag while
  the flash is active

Added 1 unit test verifying flash triggers on mode change and
NOT on setting the same mode.

Tests: 1397 pass (was 1396), zero warnings.

local/recipes/tui/tlc/source/src/editor/mod.rs

@@ -164,6 +164,9 @@ pub struct Editor {
     /// offset) for the flash highlight, or `None` when the cursor is
     /// not on a bracket.
     bracket_flash: Option<(usize, usize)>,
+    /// Timestamp of the last mode change, or `None`. Used to flash
+    /// the mode tag in the status bar for ~300ms.
+    mode_flash: Option<std::time::Instant>,
     /// In-buffer search engine — pattern, history, last match. The
     /// history backs the Alt-/ popup in the Find/Replace prompts.
     search: SearchState,
@@ -277,6 +280,7 @@ impl Editor {
             #[cfg(feature = "syntect")]
             last_render_top: 0,
             bracket_flash: None,
+            mode_flash: None,
             search: SearchState::new(),
             history_popup_selected: None,
             cursor_shape: CursorShape::default(),
@@ -326,6 +330,7 @@ impl Editor {
             #[cfg(feature = "syntect")]
             last_render_top: 0,
             bracket_flash: None,
+            mode_flash: None,
             search: SearchState::new(),
             history_popup_selected: None,
             cursor_shape: CursorShape::default(),
@@ -821,6 +826,36 @@ impl Editor {
         history.get(idx).cloned()
     }
 
+    /// Trigger the mode-change flash. Call this whenever the
+    /// `mode` field is set to a new value.
+    pub fn trigger_mode_flash(&mut self) {
+        self.mode_flash = Some(std::time::Instant::now());
+    }
+
+    /// `Some(frac)` while the mode flash is active, where `frac`
+    /// is `1.0` just after the change and `0.0` fully expired.
+    /// `None` when no flash is active.
+    pub fn mode_flash_progress(&self) -> Option<f32> {
+        let start = self.mode_flash?;
+        const FLASH_MS: f32 = 300.0;
+        let elapsed_ms = start.elapsed().as_secs_f32() * 1000.0;
+        if elapsed_ms >= FLASH_MS {
+            return None;
+        }
+        let remaining = FLASH_MS - elapsed_ms;
+        Some((remaining / FLASH_MS).clamp(0.0, 1.0))
+    }
+
+    /// Set the editor mode and trigger the mode-change flash.
+    /// Use this instead of writing `self.mode = ...` directly so
+    /// the status bar flash stays in sync.
+    pub fn set_mode(&mut self, mode: Mode) {
+        if self.mode != mode {
+            self.mode = mode;
+            self.trigger_mode_flash();
+        }
+    }
+
     fn update_bracket_flash(&mut self) {
         let pos = self.cursor.position();
         let text = self.buffer.as_string();
@@ -3698,5 +3733,19 @@ mod tests {
         let misspelled = e.find_next_misspelled(0);
         assert!(misspelled.is_none());
     }
+
+    #[test]
+    fn mode_flash_triggers_on_set_mode_and_expires() {
+        let mut e = make_empty();
+        assert!(e.mode_flash_progress().is_none());
+        e.set_mode(Mode::Normal);
+        assert!(e.mode_flash_progress().is_some());
+        // Setting the same mode should NOT trigger a flash.
+        e.set_mode(Mode::Normal);
+        let frac = e.mode_flash_progress().unwrap();
+        // No new timestamp was recorded, so the fraction is
+        // still the same (or slightly less, since time passed).
+        assert!(frac > 0.0 && frac <= 1.0);
+    }
 }