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/variable_test.c

@@ -33,7 +33,7 @@ static void allocate_page_test(void) {
 
 static void array_reference_test(void) {
 	struct VarRef ref;
-	int index = 2;
+	vmvar_t index = 2;
 
 	ASSERT_TRUE(v_allocatePage(1, 5, true));
 	ASSERT_FALSE(v_bindArray(-1, &index, 0, 1));
@@ -65,13 +65,13 @@ static void array_reference_test(void) {
 }
 
 static void string_variable_conversion_test(void) {
-	const int input[] = {
+	const vmvar_t input[] = {
 		0x41,    // 'A'
 		0xa082,  // Shift-JIS 0x82 0xa0 ('あ')
 		0x42,    // 'B'
 		0,       // terminator
 	};
-	int output[4] = {-1, -1, -1, -1};
+	vmvar_t output[4] = {-1, -1, -1, -1};
 
 	svar_fromVars(0, input);
 	ASSERT_STRCMP(svar_get(0), "A\x82\xa0" "B");