Add if statements, unary minus, and fix GC safe points

- Lexer: recognize 'if' as keyword (TOK_IF)
- Parser: add NODE_IF with if_statement union, parse if/cond/body,
  handle unary minus in parse_term as 0 - expr
- Eval: add NODE_IF evaluation, move GC to NODE_BLOCK level to avoid
  destroying temporary values during sub-expression evaluation

src/frontend/parser.h

@@ -43,6 +43,10 @@ typedef struct ASTNode {
       struct ASTNode *cond;
       struct ASTNode *body;
     } while_loop; // NODE_WHILE
+    struct {
+      struct ASTNode *cond;
+      struct ASTNode *body;
+    } if_statement; // NODE_IF
   } data;
 } ASTNode;
 
@@ -65,6 +69,15 @@ ASTNode *parse_term(Token *tokens) {
     node->data.string_val = tokens[pos].value;
     pos++;
     return node;
+  } else if (tokens[pos].type == TOK_MINUS) {
+    pos++; // consumir '-'
+    ASTNode *term = parse_term(tokens);
+    ASTNode *neg = make_node(NODE_BINOP);
+    neg->data.binop.op = '-';
+    neg->data.binop.left = make_node(NODE_INT_LIT);
+    neg->data.binop.left->data.int_val = 0;
+    neg->data.binop.right = term;
+    return neg;
   }
   printf("ERROR: esperaba INT o ID, encontré tipo %d\n", tokens[pos].type);
   exit(1);
@@ -135,6 +148,33 @@ ASTNode *parse_statement(Token *tokens) {
     node->data.while_loop.body = body;
     return node;
   }
+
+  if (tokens[pos].type == TOK_IF) {
+    pos++; // consumir if
+    ASTNode *cond = parse_expr(tokens);
+    pos++; // consumir :
+    pos++; // consumir NEWLINE
+    pos++; // consumir INDENT
+
+    // Parsear bloque de statements hasta DEDENT
+    ASTNode *body = make_node(NODE_BLOCK);
+    body->data.block.stmts = (ASTNode **)malloc(sizeof(ASTNode *) * 256);
+    body->data.block.count = 0;
+    while (tokens[pos].type != TOK_DEDENT) {
+      body->data.block.stmts[body->data.block.count++] =
+          parse_statement(tokens);
+      if (tokens[pos].type == TOK_NEWLINE) {
+        pos++;
+      }
+    }
+    pos++; // Consumir DEDENT
+
+    ASTNode *node = make_node(NODE_IF);
+    node->data.while_loop.cond = cond;
+    node->data.while_loop.body = body;
+    return node;
+  }
+
   printf("ERROR: statement inesperado\n");
   exit(1);
 }
@@ -215,6 +255,12 @@ void ast_print(ASTNode *node, const char *prefix, int is_last) {
     }
     break;
 
+  case NODE_IF:
+    printf("NODE_IF\n");
+    ast_print(node->data.while_loop.cond, new_prefix, 0);
+    ast_print(node->data.while_loop.body, new_prefix, 1);
+    break;
+
   default:
     printf("UNKNOWN\n");
     break;