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/codegen/c_backend.rs

@@ -50,6 +50,7 @@ use std::fmt::Write;
 #[derive(Debug, Clone)]
 pub struct CGenError {
     pub message: String,
+    #[allow(dead_code)]
     pub span: Option<Span>,
 }
 
@@ -96,6 +97,7 @@ pub struct CBackend {
     /// Closures to emit (collected during expression generation)
     closures: Vec<ClosureInfo>,
     /// Variables in scope (for free variable analysis)
+    #[allow(dead_code)]
     local_vars: HashSet<String>,
     /// Functions that return closures
     closure_returning_functions: HashSet<String>,
@@ -2388,6 +2390,7 @@ impl CBackend {
         }
     }
 
+    #[allow(dead_code)]
     fn unbox_value(&self, val: &str, type_hint: Option<&str>) -> String {
         match type_hint {
             Some("LuxInt") => format!("lux_unbox_int({})", val),
@@ -3323,6 +3326,7 @@ impl CBackend {
     }
 
     /// Emit decrefs for all variables in all scopes (for early return)
+    #[allow(dead_code)]
     fn emit_all_scope_cleanup(&mut self) {
         // Collect all decrefs first to avoid borrow issues
         let decrefs: Vec<String> = self.rc_scopes.iter().rev()