Add module loader and related tests

Introduce `ModuleLoader` to handle `#use` directives and dependency
resolution. This involves adding `tempfile` dependency, modifying
`Cargo.toml` and `Cargo.lock`, creating a new `module_loader.rs` file,
and updating `environment.rs` to use the new module.

Also, add comprehensive tests for `Diagnostics::format_errors`,
`CompilationResult`, and the new `ModuleLoader` functionality. These
tests cover various scenarios including empty diagnostics, multiple
errors, error formatting, compilation success/failure, and complex
dependency loading with topological ordering and search path resolution.

tests/diagnostics.rs

@@ -0,0 +1,56 @@
+use myc::ast::compiler::CompilationResult;
+use myc::ast::diagnostics::Diagnostics;
+use myc::ast::environment::Environment;
+
+// --- Diagnostics::format_errors() ---
+
+#[test]
+fn format_errors_empty() {
+    let diag = Diagnostics::new();
+    assert_eq!(diag.format_errors(), "");
+}
+
+#[test]
+fn format_errors_single() {
+    let mut diag = Diagnostics::new();
+    diag.push_error("type mismatch", None);
+    assert_eq!(diag.format_errors(), "type mismatch");
+}
+
+#[test]
+fn format_errors_multiple_joined_with_newline() {
+    let mut diag = Diagnostics::new();
+    diag.push_error("first error", None);
+    diag.push_error("second error", None);
+    assert_eq!(diag.format_errors(), "first error\nsecond error");
+}
+
+#[test]
+fn format_errors_ignores_warnings_and_hints() {
+    let mut diag = Diagnostics::new();
+    diag.push_warning("just a warning", None);
+    diag.push_hint("a hint", None);
+    diag.push_error("the real error", None);
+    // Only the error should appear
+    assert_eq!(diag.format_errors(), "the real error");
+}
+
+// --- CompilationResult ---
+
+#[test]
+fn compilation_result_error_into_err() {
+    let result = CompilationResult::error("something went wrong");
+    let err = result.into_result();
+    assert!(err.is_err());
+    assert_eq!(err.unwrap_err(), "something went wrong");
+}
+
+#[test]
+fn compilation_result_success_into_ok() {
+    // The success() constructor requires a TypedNode, which is produced by
+    // the compiler. We verify the happy path via a minimal compile round-trip.
+    let env = Environment::new();
+    let result = env.compile("(+ 1 2)");
+    assert!(!result.diagnostics.has_errors(), "unexpected errors: {}", result.diagnostics.format_errors());
+    assert!(result.into_result().is_ok());
+}