Enable ANSI rendering in Windows conhost

[Copilot]

tk emitted ANSI color sequences on Windows without enabling virtual terminal processing, so default conhost sessions displayed raw escape codes instead of colored output. This change enables the required console mode at process startup so existing ANSI output renders correctly.

• Windows console initialization

  • Enable ENABLE_VIRTUAL_TERMINAL_PROCESSING for both stdout and stderr when tk starts.
  • Skip mode updates when the handle is not a console or the flag is already set.

• Dependency surface

  • Add a Windows-only windows-sys dependency for the console API calls.
  • Keep the change isolated to the tk entrypoint; non-Windows behavior remains unchanged.

• Coverage

  • Add focused unit tests for the mode-bit helper to verify the VT flag is added without clearing existing flags.

#[cfg(windows)]
fn enable_virtual_terminal_processing() {
    for handle in [STD_OUTPUT_HANDLE, STD_ERROR_HANDLE] {
        let std_handle = unsafe { GetStdHandle(handle) };
        let mut mode = 0;
        if unsafe { GetConsoleMode(std_handle, &mut mode) } == 0 {
            continue;
        }
        let new_mode = mode | ENABLE_VIRTUAL_TERMINAL_PROCESSING;
        if new_mode != mode {
            unsafe { SetConsoleMode(std_handle, new_mode) };
        }
    }
}

• Fixes Windows: ENABLE_VIRTUAL_TERMINAL_PROCESSING flag not applied when invoked #23