feat(joinir): Phase 54 AST→JSON JoinIR-compatible statement fields

Add JoinIR-compatible fields to statement JSON output in ast_json.rs:
- Assignment: "type": "Assignment", "target" (string), "expr" alias
- Print: "type": "Print", "expr" alias
- Local: "type": "Local", "name", "expr" (for single-variable case)
- If: "type": "If", "cond" alias
- Loop: "type": "Loop", "cond" alias
- Return: "type": "Return"

This enables the stmt_handlers.rs (Phase 53) to correctly parse
statements from control_flow.rs loop body JSON.

Test results: JoinIR 56 PASS, loop_frontend_binding 7 PASS

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

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

src/macro/ast_json.rs

@@ -7,37 +7,78 @@ pub fn ast_to_json(ast: &ASTNode) -> Value {
             "kind": "Program",
             "statements": statements.into_iter().map(|s| ast_to_json(&s)).collect::<Vec<_>>()
         }),
+        // Phase 54: Loop with JoinIR-compatible fields
         ASTNode::Loop {
             condition, body, ..
         } => json!({
             "kind": "Loop",
+            "type": "Loop",  // JoinIR Frontend expects "type"
             "condition": ast_to_json(&condition),
+            "cond": ast_to_json(&condition),  // JoinIR expects "cond"
             "body": body.into_iter().map(|s| ast_to_json(&s)).collect::<Vec<_>>()
         }),
+        // Phase 54: Print with JoinIR-compatible fields
         ASTNode::Print { expression, .. } => json!({
             "kind": "Print",
+            "type": "Print",  // JoinIR Frontend expects "type"
             "expression": ast_to_json(&expression),
+            "expr": ast_to_json(&expression),  // JoinIR expects "expr"
         }),
+        // Phase 54: Return with JoinIR-compatible fields
         ASTNode::Return { value, .. } => json!({
             "kind": "Return",
+            "type": "Return",  // JoinIR Frontend expects "type"
             "value": value.as_ref().map(|v| ast_to_json(v)),
         }),
         ASTNode::Break { .. } => json!({"kind":"Break"}),
         ASTNode::Continue { .. } => json!({"kind":"Continue"}),
-        ASTNode::Assignment { target, value, .. } => json!({
+        // Phase 54: Assignment with JoinIR-compatible fields
+        ASTNode::Assignment { target, value, .. } => {
+            // Extract variable name if target is a simple Variable
+            let target_str = match target.as_ref() {
+                ASTNode::Variable { name, .. } => name.clone(),
+                _ => "complex".to_string(), // FieldAccess or other complex target
+            };
+            json!({
                 "kind": "Assignment",
-            "target": ast_to_json(&target),
+                "type": "Assignment",  // JoinIR Frontend expects "type"
+                "target": target_str,  // JoinIR expects string variable name
+                "lhs": ast_to_json(&target),  // Keep full AST for complex cases
                 "value": ast_to_json(&value),
-        }),
+                "expr": ast_to_json(&value),  // JoinIR expects "expr"
+            })
+        }
+        // Phase 54: Local with JoinIR-compatible fields
         ASTNode::Local {
             variables,
             initial_values,
             ..
-        } => json!({
+        } => {
+            // For single-variable declarations, add "name" and "expr" for JoinIR compatibility
+            let (name, expr) = if variables.len() == 1 {
+                let n = variables[0].clone();
+                let e = initial_values.get(0)
+                    .and_then(|opt| opt.as_ref())
+                    .map(|v| ast_to_json(v));
+                (Some(n), e)
+            } else {
+                (None, None)
+            };
+
+            let inits: Vec<_> = initial_values.into_iter()
+                .map(|opt| opt.map(|v| ast_to_json(&v)))
+                .collect();
+
+            json!({
                 "kind": "Local",
+                "type": "Local",  // JoinIR Frontend expects "type"
+                "name": name,  // Single variable name for JoinIR (null if multiple)
+                "expr": expr,  // Single variable init for JoinIR (null if multiple)
                 "variables": variables,
-            "inits": initial_values.into_iter().map(|opt| opt.map(|v| ast_to_json(&v))).collect::<Vec<_>>()
+                "inits": inits
-        }),
+            })
+        }
+        // Phase 54: If with JoinIR-compatible fields
         ASTNode::If {
             condition,
             then_body,
@@ -45,7 +86,9 @@ pub fn ast_to_json(ast: &ASTNode) -> Value {
             ..
         } => json!({
             "kind": "If",
+            "type": "If",  // JoinIR Frontend expects "type"
             "condition": ast_to_json(&condition),
+            "cond": ast_to_json(&condition),  // JoinIR expects "cond"
             "then": then_body.into_iter().map(|s| ast_to_json(&s)).collect::<Vec<_>>(),
             "else": else_body.map(|v| v.into_iter().map(|s| ast_to_json(&s)).collect::<Vec<_>>()),
         }),