refactor: Extract debug utilities from control_flow.rs (Phase 1)

- Created control_flow/ subdirectory
- Moved trace_varmap() to debug.rs
- All control flow logic now in control_flow/mod.rs
- Zero breaking changes, all functionality preserved
- Tests pass (binary execution verified)

src/mir/builder/control_flow/debug.rs

@@ -0,0 +1,16 @@
+//! Debug utilities for control flow tracing
+
+use super::super::MirBuilder;
+
+impl MirBuilder {
+    /// Trace variable_map state for debugging
+    /// Enable with NYASH_TRACE_VARMAP=1
+    pub(in crate::mir::builder) fn trace_varmap(&self, context: &str) {
+        if std::env::var("NYASH_TRACE_VARMAP").is_ok() {
+            let vars: Vec<_> = self.variable_map.iter()
+                .map(|(k, v)| format!("{}={:?}", k, v))
+                .collect();
+            eprintln!("[varmap/{}] {{{}}}", context, vars.join(", "));
+        }
+    }
+}