Add VM variable storage type

Define a dedicated `vmvar_t` type for values stored in variable pages
and propagate it through interfaces that expose those values. Keep the
type as int for now so this commit does not change runtime behavior.
This separates VM storage from ordinary int buffers and makes a later
conversion to 16-bit storage mechanically checkable.

src/debugger.c

@@ -142,7 +142,7 @@ static int eval_variable(void) {
 	int var = dbg_lookup_var(buf);
 	if (var < 0)
 		eval_error("unknown variable \"%s\"", buf);
-	int *store;
+	vmvar_t *store;
 	if (consume('[')) {
 		int index = eval_expr();
 		expect(']');