refactor(fspy): gate ipc_channel_conf on non-musl targets

The IPC channel sender is only used by the preload library, which is
not built on musl (only seccomp-based tracking is used there). Mark
the `ipc_channel_conf` field, its imports, and its creation site as
`#[cfg(not(target_env = "musl"))]` so musl builds don't carry the
unused configuration.

https://claude.ai/code/session_01VqiMHiGeViu1pGhWwJ67Qc

Author: claude

crates/fspy/src/unix/mod.rs

@@ -89,10 +89,15 @@ impl SpyImpl {
         #[cfg(target_os = "linux")]
         let supervisor = supervise::<SyscallHandler>().map_err(SpawnError::Supervisor)?;
 
+        #[cfg(not(target_env = "musl"))]
         let (ipc_channel_conf, ipc_receiver) =
             channel(SHM_CAPACITY).map_err(SpawnError::ChannelCreation)?;
+        #[cfg(target_env = "musl")]
+        let (_, ipc_receiver) =
+            channel(SHM_CAPACITY).map_err(SpawnError::ChannelCreation)?;
 
         let payload = Payload {
+            #[cfg(not(target_env = "musl"))]
             ipc_channel_conf,
 
             #[cfg(target_os = "macos")]

crates/fspy_shared_unix/src/payload.rs

@@ -5,10 +5,12 @@ use bincode::{Decode, Encode, config::standard};
 use bstr::BString;
 #[cfg(not(target_env = "musl"))]
 use fspy_shared::ipc::NativeStr;
+#[cfg(not(target_env = "musl"))]
 use fspy_shared::ipc::channel::ChannelConf;
 
 #[derive(Debug, Encode, Decode)]
 pub struct Payload {
+    #[cfg(not(target_env = "musl"))]
     pub ipc_channel_conf: ChannelConf,
 
     #[cfg(not(target_env = "musl"))]