Fix unit map storage for 16-bit variables

Store unit map attributes as vmvar_t and derive VR/VW transfer sizes
from their element type. This prevents buffer overruns after vmvar_t
became uint16_t and fixes upward movement in Rance IV.

Add scenario coverage for both transfer directions and verify that
adjacent variables remain intact.

test/test.adv

@@ -15,6 +15,7 @@
 	%#stack.adv:
 	%#math.adv:
 	%#variable.adv:
+	%#unitmap.adv:
 
 	HH0,tests_passed: ' tests passed' R
 	HH0,tests_failed: ' tests failed' R