Verify and fix gap types against assembler output

Assembler-verified and fixed:
- Type 12: XOP field corrected to bits10-8 (was bits11-9)
- Type 14: SF=4bits, XREG uses reg0[] not xop_shift[] (was 3-bit OOB)
- Type 16: XREG uses reg0[] (was xop_shift[], 4-bit index OOB)
- Type 19: I-reg field at bits3-2 (was bits1-0), now correct for jump(i2)
- Type 26: Push/Pop encoding is 10/11 not 01/10;
  PPP=bits6-5, LPP=bits4-3 (was bits6-4, bits3-2)
  All 6 variants verified: push/pop sts, loop, pc

Assembler-verified unchanged:
- Type 21/21a: MODIFY correct
- Type 23/24: DIVQ/DIVS correct
- Type 29: DM immediate modify read/write correct
- Type 32: Any Reg <-> DM read/write correct
- Type 34/35: IO/System register read/write correct
- Type 36: LJUMP 2-word correct
- Type 37: SETINT/CLRINT correct

Remaining note: Type 12 SF field may use a different mapping
than sf_names[] for combined shift+memory ops (ASHIFT encodes
as SF=2 not SF=4). Needs further investigation.

examples/build/verify_gaps.dsp

@@ -0,0 +1,83 @@
+/*
+ * verify_gaps.dsp -- Assembler verification for structural-only types.
+ * Tests Types 12, 14, 16, 19, 21, 21a, 22, 23, 24, 26, 29, 32, 34, 35, 36, 37.
+ */
+	.section/PM program0;
+	.global _start;
+_start:
+	/* Setup */
+	i0 = 0x0100;
+	i4 = 0x0200;
+	m0 = 1;
+	m4 = 1;
+	l0 = 0;
+	l4 = 0;
+	ax0 = 0x1234;
+	si = 0x00FF;
+
+	/* Type 12: Shift + DM read */
+	sr = lshift si (hi), mx0 = dm(i0,m0);
+
+	/* Type 12: Shift + DM write */
+	sr = ashift si (hi), dm(i0,m0) = ax0;
+
+	/* Type 14: Shift + Dreg move */
+	sr = lshift si (hi), ax0 = mx0;
+
+	/* Type 16: Conditional shift */
+	if eq sr = lshift si (hi);
+	sr = lshift si (hi);
+
+	/* Type 19: Indirect jump */
+	i2 = _after_indirect;
+	jump (i2);
+_after_indirect:
+	nop;
+
+	/* Type 21: MODIFY (reg) */
+	modify(i0,m0);
+
+	/* Type 21a: MODIFY (imm) */
+	modify(i0,5);
+
+	/* Type 23: DIVQ */
+	divq ax0;
+
+	/* Type 24: DIVS */
+	divs ay0, ax0;
+
+	/* Type 26: Push/Pop */
+	push sts;
+	pop sts;
+	push loop;
+	pop loop;
+	push pc;
+	pop pc;
+
+	/* Type 29: DM immediate modify */
+	ax0 = dm(i0,1);
+	dm(i0,1) = ax0;
+
+	/* Type 32: Any Reg <-> DM (postmodify) */
+	ar = dm(i0,m0);
+	dm(i0,m0) = ar;
+
+	/* Type 34: IO */
+	ax0 = io(0x00);
+	io(0x00) = ax0;
+
+	/* Type 35: System register */
+	ax0 = reg(0x00);
+	reg(0x00) = ax0;
+
+	/* Type 37: SETINT/CLRINT */
+	setint 3;
+	clrint 3;
+
+	/* Type 36: Long jump */
+	ljump _end;
+
+_end:
+	nop;
+_halt:
+	jump _halt;