feat: C backend module import support, Int/Float.toString, Test.assertEqualMsg

The C backend can now compile programs that import user-defined modules.
Module-qualified calls like `mymodule.func(args)` are resolved to prefixed
C functions (e.g., `mymodule_func_lux`), with full support for transitive
imports and effect-passing. Also adds Int.toString/Float.toString to type
system, interpreter, and C backend, and Test.assertEqualMsg for labeled
test assertions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/modules.rs

@@ -305,6 +305,11 @@ impl ModuleLoader {
         self.cache.iter()
     }
 
+    /// Get the module cache (for passing to C backend)
+    pub fn module_cache(&self) -> &HashMap<String, Module> {
+        &self.cache
+    }
+
     /// Clear the module cache
     pub fn clear_cache(&mut self) {
         self.cache.clear();