feat(joinir): Phase 50 Loop Frontend Binding layer

Phase 50 implements the Loop Frontend Binding layer that maps
actual loop variables to JoinIR Frontend's expected names (i, acc, n).

## Changes

- Add loop_frontend_binding.rs module with:
  - LoopFrontendBinding struct for variable mapping
  - for_print_tokens() and for_array_filter() factory methods
  - generate_local_declarations() for JSON v0 format
  - rename_body_variables() for out → acc renaming

- Integrate binding with cf_loop_joinir_impl:
  - Create binding based on function name
  - Inject i/acc/n Local declarations into JSON v0
  - Use correct JoinIR Frontend type names (Int/Var/Method)

## Limitations Found

JoinIR Frontend doesn't support:
- Field access (me.tokens) - blocks print_tokens
- NewBox (new ArrayBox()) - blocks array_filter

These will be addressed in Phase 51.

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

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

src/mir/builder/control_flow.rs

@@ -132,17 +132,50 @@ impl super::MirBuilder {
         func_name: &str,
         debug: bool,
     ) -> Result<Option<ValueId>, String> {
+        use super::loop_frontend_binding::LoopFrontendBinding;
         use crate::r#macro::ast_json::ast_to_json;
         use crate::mir::join_ir::frontend::{AstToJoinIrLowerer, JoinFuncMetaMap};
         use crate::mir::join_ir_vm_bridge::convert_join_module_to_mir_with_meta;
         use crate::mir::types::ConstValue;
 
+        // Phase 50: Create appropriate binding based on function name
+        let binding = match func_name {
+            "JsonTokenizer.print_tokens/0" => LoopFrontendBinding::for_print_tokens(),
+            "ArrayExtBox.filter/2" => LoopFrontendBinding::for_array_filter(),
+            _ => {
+                if debug {
+                    eprintln!(
+                        "[cf_loop/joinir] No binding defined for {}, falling back",
+                        func_name
+                    );
+                }
+                return Ok(None);
+            }
+        };
+
+        if debug {
+            eprintln!(
+                "[cf_loop/joinir] Using binding: counter={}, acc={:?}, pattern={:?}",
+                binding.counter_var,
+                binding.accumulator_var,
+                binding.pattern
+            );
+        }
+
         // Step 1: Convert condition and body to JSON
         let condition_json = ast_to_json(condition);
-        let body_json: Vec<serde_json::Value> = body.iter().map(|s| ast_to_json(s)).collect();
+        let mut body_json: Vec<serde_json::Value> =
+            body.iter().map(|s| ast_to_json(s)).collect();
+
+        // Phase 50: Rename variables in body (e.g., "out" → "acc" for filter)
+        binding.rename_body_variables(&mut body_json);
+
+        // Phase 50: Generate Local declarations from binding
+        let (i_local, acc_local, n_local) = binding.generate_local_declarations();
 
         // Step 2: Construct JSON v0 format with "defs" array
         // The function is named "simple" to match JoinIR Frontend's pattern matching
+        // Phase 50: Include i/acc/n Local declarations to satisfy JoinIR Frontend expectations
         let program_json = serde_json::json!({
             "defs": [
                 {
@@ -151,16 +184,19 @@ impl super::MirBuilder {
                     "body": {
                         "type": "Block",
                         "body": [
-                            // Placeholder locals (JoinIR Frontend will infer from loop)
+                            // Phase 50: Inject i/acc/n Local declarations
+                            i_local,
+                            acc_local,
+                            n_local,
                             {
                                 "type": "Loop",
                                 "condition": condition_json,
                                 "body": body_json
                             },
-                            // Placeholder return
+                            // Return the accumulator (or null for side-effect loops)
                             {
                                 "type": "Return",
-                                "value": null
+                                "value": { "kind": "Variable", "name": "acc" }
                             }
                         ]
                     }