fix(abi): exclude internal Apple recvmsg_x FFI from the C header

cbindgen swept transport/udp.rs's `recvmsg_x` foreign import and its `MsghdrX`
#[repr(C)] struct into the generated C header — they're internal Apple-only FFI,
not part of the public C ABI, and reference socklen_t/ssize_t/iovec which the C
ABI harness doesn't include, so c_abi_harness_round_trips failed to compile.
Add them to cbindgen.toml export.exclude and regenerate the header.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

include/punktfunk_core.h

@@ -470,18 +470,6 @@ typedef struct {
     float loss_pct;
 } PunktfunkProbeResult;
 
-// `struct msghdr_x` from Darwin `<sys/socket.h>` (the batched-I/O variant — not in the `libc` crate).
-typedef struct {
-    void *msg_name;
-    socklen_t msg_namelen;
-    iovec *msg_iov;
-    int msg_iovlen;
-    void *msg_control;
-    socklen_t msg_controllen;
-    int msg_flags;
-    size_t msg_datalen;
-} MsghdrX;
-
 #ifdef __cplusplus
 extern "C" {
 #endif // __cplusplus
@@ -899,10 +887,6 @@ PunktfunkStatus punktfunk_connection_probe_result(const PunktfunkConnection *c,
 void punktfunk_connection_close(PunktfunkConnection *c);
 #endif
 
-// Darwin batched receive: up to `cnt` datagrams in one syscall; returns the count received and
-// sets each `msg_datalen` to its byte length. Present in libSystem on all macOS/iOS.
-extern ssize_t recvmsg_x(int s, MsghdrX *msgp, unsigned int cnt, int flags);
-
 #ifdef __cplusplus
 }  // extern "C"
 #endif  // __cplusplus