feat(builder): Phase 49 cf_loop JoinIR Frontend mainline integration point

Add try_cf_loop_joinir() method to control_flow.rs as the routing point
for JoinIR Frontend mainline integration.

- First target: JsonTokenizer.print_tokens/1
- Controlled by HAKO_JOINIR_PRINT_TOKENS_MAIN=1 flag
- Currently falls through to legacy LoopBuilder (Phase 49-3 will implement)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/mir/builder/control_flow.rs

@@ -21,11 +21,23 @@ impl super::MirBuilder {
     }
 
     /// Control-flow: loop
+    ///
+    /// # Phase 49: JoinIR Frontend Mainline Integration
+    ///
+    /// This is the unified entry point for all loop lowering. When enabled via
+    /// `HAKO_JOINIR_PRINT_TOKENS_MAIN=1`, specific functions (starting with
+    /// `JsonTokenizer.print_tokens/1`) are routed through JoinIR Frontend instead
+    /// of the traditional LoopBuilder path.
     pub(super) fn cf_loop(
         &mut self,
         condition: ASTNode,
         body: Vec<ASTNode>,
     ) -> Result<ValueId, String> {
+        // Phase 49: Try JoinIR Frontend route for mainline targets
+        if let Some(result) = self.try_cf_loop_joinir(&condition, &body)? {
+            return Ok(result);
+        }
+
         if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
             eprintln!("[cf_loop] CALLED from somewhere");
             eprintln!("[cf_loop] Current stack (simulated): check build_statement vs build_expression_impl");
@@ -35,6 +47,50 @@ impl super::MirBuilder {
         loop_builder.build_loop(condition, body)
     }
 
+    /// Phase 49: Try JoinIR Frontend for mainline integration
+    ///
+    /// Returns `Ok(Some(value))` if the current function should use JoinIR Frontend,
+    /// `Ok(None)` to fall through to the legacy LoopBuilder path.
+    fn try_cf_loop_joinir(
+        &mut self,
+        _condition: &ASTNode,
+        _body: &[ASTNode],
+    ) -> Result<Option<ValueId>, String> {
+        // Phase 49-2: Check if feature is enabled
+        if std::env::var("HAKO_JOINIR_PRINT_TOKENS_MAIN").ok().as_deref() != Some("1") {
+            return Ok(None);
+        }
+
+        // Get current function name
+        let func_name = self
+            .current_function
+            .as_ref()
+            .map(|f| f.signature.name.as_str())
+            .unwrap_or("");
+
+        // Phase 49-2: Only handle print_tokens for now
+        if func_name != "JsonTokenizer.print_tokens/1" {
+            return Ok(None);
+        }
+
+        // Debug log when routing through JoinIR Frontend
+        if std::env::var("NYASH_LOOPFORM_DEBUG").is_ok() {
+            eprintln!(
+                "[cf_loop/joinir] Routing {} through JoinIR Frontend mainline",
+                func_name
+            );
+        }
+
+        // TODO(Phase 49-3): Implement actual JoinIR Frontend integration
+        // 1. Convert condition + body to AST JSON (Program)
+        // 2. Call AstToJoinIrLowerer::lower_program_json()
+        // 3. Convert JoinModule to MIR via convert_join_module_to_mir_with_meta()
+        // 4. Merge generated blocks into current_function
+        //
+        // For now, fall through to legacy path
+        Ok(None)
+    }
+
     /// Control-flow: try/catch/finally
     pub(super) fn cf_try_catch(
         &mut self,