feat(joinir): Phase 52-53 LoopFrontendBinding JSON + Statement Handlers

Phase 52: LoopFrontendBinding JSON generation fixes
- Add receiver_to_json() for Field node structure (me.tokens)
- Add needs_me_receiver() for instance method detection
- Fix "condition" → "cond" key for JoinIR Frontend
- Add me parameter propagation in loop_patterns.rs
- Add JoinIR-compatible type fields in ast_json.rs
  - Variable → "type": "Var"
  - Literal → "type": "Int"/"Bool" (literal_to_joinir_json)
  - BinaryOp → "type": "Binary"/"Compare" (is_compare_op)
  - MethodCall → "type": "Method"

Phase 53: Statement Handler module for loop body
- NEW: stmt_handlers.rs with StatementEffect type
- Support: Local, Assignment, Print, Method, If statements
- If lowering: single variable update → Select instruction
- Remove hardcoded assert in loop_patterns.rs
- Replace with generic lower_statement() calls

Test results: 56 JoinIR tests PASS, 7 loop_frontend_binding tests PASS

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

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

src/mir/join_ir/frontend/ast_lowerer/mod.rs

@@ -32,6 +32,7 @@ mod if_return;
 mod loop_patterns;
 mod nested_if;
 mod read_quoted;
+mod stmt_handlers;
 
 #[cfg(test)]
 mod tests;