Add string type support: literals, concatenation, println, and if eval

- Implement string literal tokenization and parsing (lexer + parser)
- Add string concatenation with + operator in evaluator
- Add println keyword for printing with newline
- Add NODE_IF evaluation in VM
- Fix null terminator bug in string concat buffer

src/frontend/parser.h

@@ -13,9 +13,10 @@ typedef enum {
   NODE_ASSIGN,     // asignacion: x = expr
   NODE_BINOP,      // operacion binaria: a + b
   NODE_PRINT,      // print(expr)
-  NODE_IF,         // if cond: bloque
-  NODE_WHILE,      // while cond: bloque
-  NODE_BLOCK,      // secuencia de statements
+  NODE_PRINTLN,
+  NODE_IF,    // if cond: bloque
+  NODE_WHILE, // while cond: bloque
+  NODE_BLOCK, // secuencia de statements
 } NodeType;
 
 typedef struct ASTNode {
@@ -64,6 +65,11 @@ ASTNode *parse_term(Token *tokens) {
     node->data.int_val = atoi(tokens[pos].value);
     pos++;
     return node;
+  } else if (tokens[pos].type == TOK_STRING) {
+    ASTNode *node = make_node(NODE_STRING_LIT);
+    node->data.string_val = tokens[pos].value;
+    pos++;
+    return node;
   } else if (tokens[pos].type == TOK_ID) {
     ASTNode *node = make_node(NODE_VAR);
     node->data.string_val = tokens[pos].value;
@@ -79,7 +85,7 @@ ASTNode *parse_term(Token *tokens) {
     neg->data.binop.right = term;
     return neg;
   }
-  printf("ERROR: esperaba INT o ID, encontré tipo %d\n", tokens[pos].type);
+  printf("ERROR: esperaba INT o ID, encontre tipo %d\n", tokens[pos].type);
   exit(1);
 }
 
@@ -123,6 +129,15 @@ ASTNode *parse_statement(Token *tokens) {
     return node;
   }
 
+  if (tokens[pos].type == TOK_PRINTLN) {
+    pos++; // consumir "println"
+    ASTNode *expr = parse_expr(tokens);
+
+    ASTNode *node = make_node(NODE_PRINTLN);
+    node->data.print.expr = expr;
+    return node;
+  }
+
   if (tokens[pos].type == TOK_WHILE) {
     pos++; // consumir while
     ASTNode *cond = parse_expr(tokens);
@@ -247,6 +262,11 @@ void ast_print(ASTNode *node, const char *prefix, int is_last) {
     ast_print(node->data.print.expr, new_prefix, 1);
     break;
 
+  case NODE_PRINTLN:
+    printf("NODE_PRINTLN\n");
+    ast_print(node->data.print.expr, new_prefix, 1);
+    break;
+
   case NODE_BLOCK:
     printf("NODE_BLOCK\n");
     for (int i = 0; i < node->data.block.count; i++) {