feat(runtime): add compile-time check for mutually exclusive features

Add compile_error! checks to prevent both `tokio` and `smol` features
from being enabled simultaneously in libp2p-swarm, libp2p-tcp, and
libp2p-quic crates.

This follows Cargo's recommended approach for handling mutually exclusive
features as documented in:
https://doc.rust-lang.org/cargo/reference/features.html#mutually-exclusive-features

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: neotheprogramist

swarm/src/lib.rs

@@ -53,6 +53,14 @@
 
 #![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
 
+// Ensure `tokio` and `smol` features are not enabled at the same time.
+// See: https://doc.rust-lang.org/cargo/reference/features.html#mutually-exclusive-features
+#[cfg(all(feature = "tokio", feature = "smol"))]
+compile_error!(
+    "features `tokio` and `smol` are mutually exclusive; \
+     please enable only one async runtime feature"
+);
+
 mod connection;
 mod executor;
 mod stream;

transports/quic/src/lib.rs

@@ -60,6 +60,14 @@
 
 #![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
 
+// Ensure `tokio` and `smol` features are not enabled at the same time.
+// See: https://doc.rust-lang.org/cargo/reference/features.html#mutually-exclusive-features
+#[cfg(all(feature = "tokio", feature = "smol"))]
+compile_error!(
+    "features `tokio` and `smol` are mutually exclusive; \
+     please enable only one async runtime feature"
+);
+
 mod config;
 mod connection;
 mod hole_punching;

transports/tcp/src/lib.rs

@@ -28,6 +28,14 @@
 
 #![cfg_attr(docsrs, feature(doc_cfg, doc_auto_cfg))]
 
+// Ensure `tokio` and `smol` features are not enabled at the same time.
+// See: https://doc.rust-lang.org/cargo/reference/features.html#mutually-exclusive-features
+#[cfg(all(feature = "tokio", feature = "smol"))]
+compile_error!(
+    "features `tokio` and `smol` are mutually exclusive; \
+     please enable only one async runtime feature"
+);
+
 mod provider;
 
 use std::{