fix: Add platform guards for WASM import symbols to fix Windows linking

Fixes freenet/river#39

Windows MSVC linker requires all external symbols to be resolved, even for
cdylib crates. The WASM import symbols (__frnt__logger__info,
__frnt__rand__rand_bytes, __frnt__time__utc_now) are only valid when
compiling to wasm32-unknown-unknown target.

This change:
- Guards WASM imports with #[cfg(target_family = "wasm")]
- Provides stub implementations for non-WASM targets
- Allows contracts to compile on Windows native target

The stub implementations are never used at runtime since contracts only
execute in the Freenet WASM runtime.

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

Co-Authored-By: Claude <[email protected]>

Author: sanity

rust/src/log.rs

@@ -5,8 +5,16 @@ pub fn info(msg: &str) {
     }
 }
 
+#[cfg(target_family = "wasm")]
 #[link(wasm_import_module = "freenet_log")]
 extern "C" {
     #[doc(hidden)]
     fn __frnt__logger__info(id: i64, ptr: i64, len: i32);
 }
+
+#[cfg(not(target_family = "wasm"))]
+#[allow(non_snake_case)]
+unsafe fn __frnt__logger__info(_id: i64, _ptr: i64, _len: i32) {
+    // Stub implementation for non-WASM targets (e.g., Windows native compilation)
+    // These contracts are meant to run only in WASM runtime
+}

rust/src/rand.rs

@@ -33,8 +33,16 @@ pub fn rand_bytes(number: u32) -> Vec<u8> {
     }
 }
 
+#[cfg(target_family = "wasm")]
 #[link(wasm_import_module = "freenet_rand")]
 extern "C" {
     #[doc(hidden)]
     fn __frnt__rand__rand_bytes(id: i64, ptr: i64, len: u32);
 }
+
+#[cfg(not(target_family = "wasm"))]
+#[allow(non_snake_case)]
+unsafe fn __frnt__rand__rand_bytes(_id: i64, _ptr: i64, _len: u32) {
+    // Stub implementation for non-WASM targets (e.g., Windows native compilation)
+    // These contracts are meant to run only in WASM runtime
+}

rust/src/time.rs

@@ -14,8 +14,16 @@ pub fn now() -> DateTime<Utc> {
     }
 }
 
+#[cfg(target_family = "wasm")]
 #[link(wasm_import_module = "freenet_time")]
 extern "C" {
     #[doc(hidden)]
     fn __frnt__time__utc_now(id: i64, ptr: i64);
 }
+
+#[cfg(not(target_family = "wasm"))]
+#[allow(non_snake_case)]
+unsafe fn __frnt__time__utc_now(_id: i64, _ptr: i64) {
+    // Stub implementation for non-WASM targets (e.g., Windows native compilation)
+    // These contracts are meant to run only in WASM runtime
+}