chore(gates): zero clippy warnings and a buildable sandbox example on Windows

The full-workspace gates had never run on this platform: collapsible
ifs in the Windows shell probe, a same-type cast in the crash handler,
an unused AsRawHandle import, and the sandbox smoke-test example
building its unix-only body (libc, kernel enforcement) unconditionally.
The example now stubs main off-unix; formatting reflow from cargo fmt.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KSy9S31pZNWXtRZNVJ1sDf

crates/codegen/kigi-tty-utils/src/lib.rs

@@ -567,7 +567,7 @@ pub fn dup_tui_stderr() -> io::Result<std::fs::File> {
         // `DuplicateHandle` — avoiding the `from_raw_handle` footgun
         // where `File` would take ownership of the process stderr handle
         // and close it on drop.
-        use std::os::windows::io::{AsRawHandle, FromRawHandle};
+        use std::os::windows::io::FromRawHandle;
         let stderr_handle = unsafe {
             windows::Win32::System::Console::GetStdHandle(
                 windows::Win32::System::Console::STD_ERROR_HANDLE,