Fix Constant-YOP table, Type 18, truncation guards

Type 9 Constant-YOP: Implement exact constant table from docs.
  Index = (YY<<2)|CC, value = 1<<idx for BO=01, -(1<<idx)-1
  for BO=11.  Special case idx=15: -32768 / +32767.
  (was: simple cc with optional negate, completely wrong values)

Type 18: Fix DIS encoding to check for bit pattern 10 (was 01).
  Verified all 8 modes against assembler: ENA/DIS for
  AS, MM, BR, TI all correct.

Truncation guards: Use saved 'avail' from original op->size
  instead of checking op->size after it was set to 3.
  (was: always true, guards never triggered)

examples/build/type18_test.dsp

@@ -0,0 +1,17 @@
+/*
+ * type18_test.dsp -- Verify Type 18 mode control encoding.
+ */
+	.section/PM program0;
+	.global _start;
+_start:
+	ena ar_sat;
+	dis ar_sat;
+	ena m_mode;
+	dis m_mode;
+	ena bit_rev;
+	dis bit_rev;
+	ena timer;
+	dis timer;
+	nop;
+_halt:
+	jump _halt;