v0.12.0: Rust 2024 edition migration

- edition = "2024" bump.
- FFI block in src/scsi/macos.rs wrapped in `unsafe extern "C" { }`.
- vtable_fn body gets an explicit unsafe block (unsafe_op_in_unsafe_fn).
- Match-ergonomics cleanup in mux/meta.rs, mkvstream.rs, network.rs,
  stdio.rs — removed redundant `ref` / `ref mut` bindings.

MSRV unchanged at 1.86. 226 tests pass. No behavior change.

src/mux/network.rs

@@ -81,7 +81,7 @@ impl crate::pes::Stream for NetworkStream {
         match &mut self.mode {
             Mode::Write {
                 writer,
-                ref mut header_written,
+                header_written,
                 ..
             } => {
                 if !*header_written {