feat: add File.copy and propagate effectful callback effects (WISH-7, WISH-14)

File.copy(source, dest) copies files via interpreter (std::fs::copy) and
C backend (fread/fwrite). Effectful callbacks passed to higher-order
functions like List.map/forEach now propagate their effects to the
enclosing function's inferred effect set.

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

src/interpreter.rs

@@ -3864,6 +3864,30 @@ impl Interpreter {
                 }
             }
 
+            ("File", "copy") => {
+                let source = match request.args.first() {
+                    Some(Value::String(s)) => s.clone(),
+                    _ => return Err(RuntimeError {
+                        message: "File.copy requires a string source path".to_string(),
+                        span: None,
+                    }),
+                };
+                let dest = match request.args.get(1) {
+                    Some(Value::String(s)) => s.clone(),
+                    _ => return Err(RuntimeError {
+                        message: "File.copy requires a string destination path".to_string(),
+                        span: None,
+                    }),
+                };
+                match std::fs::copy(&source, &dest) {
+                    Ok(_) => Ok(Value::Unit),
+                    Err(e) => Err(RuntimeError {
+                        message: format!("Failed to copy '{}' to '{}': {}", source, dest, e),
+                        span: None,
+                    }),
+                }
+            }
+
             // ===== Process Effect =====
             ("Process", "exec") => {
                 use std::process::Command;