tui: real terminal cursor in composer + window title (v2)

Two v2-unlocked polish items now that the TUI is on bubbletea v2:

Native cursor — the composer caret was a frame-animated "▏" that forced the
redraw loop to keep ticking in insert mode. v2's View.Cursor lets the terminal
draw and blink a real cursor, so:
- editorLines/launcherInput drop a width-1 marker rune (U+E123, same cell width
  as "▏") at the caret; View() locates it in the fully-composited frame via
  splitCursor → (X,Y), swaps it for a space, and sets a blinking CursorBar there.
- render() (preview/golden tests) resolves the marker to the static "▏" glyph, so
  screenshots are unchanged and the marker never leaks into output.
- nil cursor in normal mode hides it (vim-correct); gated on overlay==None so an
  open palette/files modal (which keeps editMode==Insert) doesn't draw a cursor
  behind it.
- animating() no longer forces a redraw in insert mode — the terminal blinks the
  cursor itself, so an idle insert screen holds still (native text selection
  survives) and burns no frames.

Window title — View.WindowTitle names the terminal tab after the active session,
falling back to the model name then "correx".

Tests: new cursor_test.go (coord extraction, mode/overlay gating, no marker leak);
updated the animating-gate test for the new insert-mode contract. Build, vet,
gofmt, full suite green; compose preview shows the static caret with no marker leak.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/tui-go/internal/app/update.go

@@ -48,7 +48,10 @@ func (m Model) Init() tea.Cmd {
 // When false, the tick loop stops and the screen holds still — so a native terminal
 // text selection survives instead of being wiped by the next redraw.
 func (m Model) animating() bool {
-	if m.editMode == ModeInsert || m.steering || m.clarTyping || m.proposeTyping || m.configEditing {
+	// Note: ModeInsert is intentionally NOT here — the composer caret is now a real
+	// terminal cursor (View.Cursor) that the terminal blinks itself, so insert mode no
+	// longer needs the frame loop. The other typing surfaces keep their drawn carets.
+	if m.steering || m.clarTyping || m.proposeTyping || m.configEditing {
 		return true
 	}
 	if m.reconnecting {