Handle’s std::os::unix::io::IntoRawFd implementation panics on stdin/stdout/stderr

This panics:

```rust
std::os::unix::io::IntoRawFd::into_raw_fd(same_file::Handle::stdin().unwrap())
````

The `Option` field is treated carelessly, and so the reassuring “unwrapping won’t panic” comment in the `Drop` implementation is wrong.

Really, `Option` is the wrong tool to use here; it should be `ManuallyDrop` these days (✓ available within the current MSRV).

Start by locating the Handle implementation and its std::os::unix::io::IntoRawFd and Drop implementations. Read how the Option field is handled for stdin, stdout, and stderr, then verify that replacing the ownership approach with ManuallyDrop prevents the conversion and cleanup paths from panicking while preserving the current MSRV.

Written by the indexing model from the issue text.