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/main.rs

@@ -174,9 +174,14 @@ fn main() {
                     .and_then(|s| s.parse::<u16>().ok())
                     .unwrap_or(8080);
 
-                let dir = args.get(2)
-                    .filter(|a| !a.starts_with('-'))
-                    .map(|s| s.as_str())
+                let port_value_idx = args.iter()
+                    .position(|a| a == "--port" || a == "-p")
+                    .map(|i| i + 1);
+                let dir = args.iter().enumerate()
+                    .skip(2)
+                    .filter(|(i, a)| !a.starts_with('-') && Some(*i) != port_value_idx)
+                    .map(|(_, a)| a.as_str())
+                    .next()
                     .unwrap_or(".");
 
                 serve_static_files(dir, port);
@@ -842,7 +847,7 @@ fn compile_to_c(path: &str, output_path: Option<&str>, run_after: bool, emit_c:
 
     // Generate C code
     let mut backend = CBackend::new();
-    let c_code = match backend.generate(&program) {
+    let c_code = match backend.generate(&program, loader.module_cache()) {
         Ok(code) => code,
         Err(e) => {
             eprintln!("{} C codegen: {}", c(colors::RED, "error:"), e);