Stabilize IME candidate positioning in Tabby

[Copilot]

Tabby’s Electron/xterm.js renderer can sample intermediate cursor positions during rapid TUI redraws, causing Chinese IME candidate windows to jump or drift.

• Tabby-specific rendering safeguards
  • Detect TERM_PROGRAM=Tabby
  • Enable low-motion rendering and bounded redraw cadence
  • Disable automatic DEC synchronized output, which can expose unstable intermediate cursor state
• Regression coverage
  • Verify Tabby enables calm rendering and turns synchronized output off automatically

if term_program.contains("tabby") {
    self.low_motion = true;
    self.fancy_animations = false;
    self.constrained_frame_rate = true;

    if self.synchronized_output.eq_ignore_ascii_case("auto") {
        self.synchronized_output = "off".to_string();
    }
}

• Fixes # Bug Report: IME Candidate Window Jumps / Unstable Position During Input #5023