Add evaluator with environment and file-based execution

Implements recursive AST evaluator with variable environment,
reads .j files from command line args, and executes programs
end-to-end (lexer -> parser -> eval).

src/vm/eval.h

@@ -0,0 +1,86 @@
+#include "../objects/object.h"
+#include "../frontend/parser.h"
+
+typedef struct
+{
+    char *name;
+    size_t value; // offset al Object en el allocator
+} Variable;
+
+typedef struct
+{
+    Variable vars[256];
+    int count;
+} Environment;
+
+size_t env_get(Environment *env, const char *name)
+{
+    for (int i = 0; i < env->count; i++)
+    {
+        if (strcmp(env->vars[i].name, name) == 0)
+        {
+            return env->vars[i].value;
+        }
+    }
+
+    printf("ERROR: variable '%s' no definida\n", name);
+    exit(1);
+}
+
+void env_set(Environment *env, const char *name, size_t value)
+{
+    for (int i = 0; i < env->count; i++)
+    {
+        if (strcmp(env->vars[i].name, name) == 0)
+        {
+            env->vars[i].value = value;
+            return;
+        }
+    }
+
+    // No existe, añadir
+    env->vars[env->count].name = (char *)name;
+    env->vars[env->count].value = value;
+    env->count++;
+}
+
+size_t eval(ASTNode *node, Environment *env, void *allocator) {
+    switch (node->type) {
+        case NODE_INT_LIT:
+            return obj_new_int(allocator, node->data.int_val);
+        case NODE_VAR:
+            return env_get(env, node->data.string_val);
+        case NODE_ASSIGN: {
+            size_t val = eval(node->data.assign.value, env, allocator);
+            env_set(env, node->data.assign.name, val);
+            return val;
+        }
+        case NODE_BINOP: {
+            size_t left = eval(node->data.binop.left, env, allocator);
+            size_t right = eval(node->data.binop.right, env, allocator);
+
+            // Resolver objects
+            Object* l = (Object *) JLANG_RESOLVE(allocator, left);
+            Object* r = (Object *) JLANG_RESOLVE(allocator, right);
+
+            // Operar (ints por ahora)
+            if (node->data.binop.op == '+'){
+                return obj_new_int(allocator, l->data.int_val + r->data.int_val);
+            } else if (node->data.binop.op == '-') {
+                return obj_new_int(allocator, l->data.int_val - r->data.int_val);
+            }
+        }
+        case NODE_PRINT: {
+            size_t val = eval(node->data.print.expr, env, allocator);
+            obj_print(allocator, val, "");
+            printf("\n");
+            return val;
+        }        
+        case NODE_BLOCK:
+            for (int i=0; i< node->data.block.count; i++)
+                eval(node->data.block.stmts[i], env, allocator);
+            return 0;
+        default:
+            break;
+    }
+}