fix: resolve all cargo compiler warnings

- Fix unused import std::io::Read in interpreter.rs by using qualified call
- Add #[allow(dead_code)] to CGenError.span (kept for future error reporting)
- Add #[allow(dead_code)] to local_vars field (planned for free variable analysis)
- Add #[allow(dead_code)] to unbox_value and emit_all_scope_cleanup methods

All 263 tests pass.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/interpreter.rs

@@ -3442,11 +3442,7 @@ impl Interpreter {
 
                         // Read body
                         let mut body = String::new();
-                        {
-                            use std::io::Read;
-                            let reader = request.as_reader();
-                            let _ = reader.read_to_string(&mut body);
-                        }
+                        let _ = std::io::Read::read_to_string(&mut request.as_reader(), &mut body);
 
                         // Extract headers
                         let headers: Vec<Value> = request.headers()