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>

crates/punktfunk-core/cbindgen.toml

@@ -12,6 +12,12 @@ documentation_style = "c99"
 [parse]
 parse_deps = false
 
+[export]
+# Internal Apple-only FFI (transport/udp.rs `recvmsg_x` batched recv + its `MsghdrX`) — NOT part of
+# the C ABI. cbindgen otherwise sweeps the foreign import and its #[repr(C)] struct into the header,
+# where socklen_t/ssize_t/iovec are undefined and the C harness fails to compile.
+exclude = ["MsghdrX", "recvmsg_x"]
+
 [export.rename]
 "InputEvent" = "PunktfunkInputEvent"
 "InputKind" = "PunktfunkInputKind"