fix(core): libc is a unix-wide dep — unbreak iOS/tvOS xcframework slices

6b5ee9f added a libc-based batched recv_batch for the Apple/BSD targets
(cfg(all(unix, not(target_os = "linux")))) but left libc declared only under
cfg(target_os = "linux"). The macOS host build pulls libc in transitively so it
compiled, but the iOS/tvOS cross-compiles (no transitive libc, dev-deps off) failed
with E0433 "cannot find crate libc", breaking the full xcframework build. Widen the
gate to cfg(unix): libc is now used by sendmmsg/recvmmsg on Linux AND recv() on the
other unix (Apple/BSD) targets.

Verified: cargo build --release -p punktfunk-core --features quic for
aarch64-apple-ios, x86_64-apple-ios, and aarch64-apple-tvos (-Z build-std) all link.

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

crates/punktfunk-core/Cargo.toml

@@ -46,9 +46,12 @@ hmac = { version = "0.12", optional = true }
 spake2 = { version = "0.4", optional = true }
 tokio = { version = "1", optional = true, features = ["rt-multi-thread", "net", "sync", "macros"] }
 
-# `sendmmsg` batched UDP send (the 1 Gbps+ syscall lever) — Linux only; other targets use the
-# scalar `send` loop fallback.
-[target.'cfg(target_os = "linux")'.dependencies]
+# `libc` for batched UDP syscalls: `sendmmsg`/`recvmmsg` on Linux (the 1 Gbps+ lever) and the
+# `recv(MSG_DONTWAIT)` drain on the other unix (Apple/BSD) targets, which have no `recvmmsg`
+# (see transport/udp.rs `recv_batch`). Needed on every unix target — non-unix (Windows) uses
+# the scalar fallbacks. Cross-compiles (iOS/tvOS) don't pull libc transitively the way the
+# macOS host build does, so it must be a direct dep here or those slices fail to link `libc::`.
+[target.'cfg(unix)'.dependencies]
 libc = "0.2"
 
 [dev-dependencies]