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(", "));
+        }
+    }
+}

src/mir/builder/control_flow/mod.rs

@@ -1,19 +1,18 @@
 //! Control-flow entrypoints (if/loop/try/throw) centralized here.
+//!
+//! This module is being modularized in phases:
+//! - Phase 1: Debug utilities
+//! - Phase 2: Pattern lowerers
+//! - Phase 3: JoinIR routing
+//! - Phase 4-19: Additional modularization (future)
+
 use super::{Effect, EffectMask, MirInstruction, ValueId};
 use crate::ast::ASTNode;
 
-impl super::MirBuilder {
+// Phase 1: Debug utilities
-    /// Trace variable_map state for debugging
+pub(in crate::mir::builder) mod debug;
-    /// Enable with NYASH_TRACE_VARMAP=1
-    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(", "));
-        }
-    }
 
+impl super::MirBuilder {
     /// Control-flow: block
     pub(super) fn cf_block(&mut self, statements: Vec<ASTNode>) -> Result<ValueId, String> {
         // identical to build_block; kept here for future policy hooks