Fix Type 1 AMF bitfield and add real FIR/IIR test ROMs

- Fix Type 1 multifunction: AMF=bits17-13, DD=bits19-18, PD=bits21-20
  (was off by one bit, causing wrong MAC decode on real firmware)
- Add FIR filter ROM (fir.bin): 52 instructions, assembled from AD example
- Add IIR biquad ROM (iir.bin): 28 instructions, assembled from AD example
- Add open21xx-compatible source files (build/fir.dsp, build/iir.dsp)
- Both ROMs disassemble correctly showing expected DSP patterns:
  MAC+dual-read kernels, DO UNTIL loops, ASHIFT scaling, RTS(DB)
- Full regression: isa_test.bin unchanged

examples/build/iir.dsp

@@ -0,0 +1,41 @@
+/*
+ * iir.dsp -- ADSP-2191 IIR biquad filter for open21xx.
+ * Based on Analog Devices application note.
+ */
+	.section/PM program0;
+	.global _start;
+_start:
+	/* Setup */
+	i0 = 0x0100;	/* delay line */
+	i1 = 0x0200;	/* input */
+	i2 = 0x0300;	/* output */
+	i4 = 0x0400;	/* coefficients (PM) */
+	l0 = 4;	/* 2 * biquad_secs */
+	l4 = 8;	/* 4 coeffs per biquad * 2 */
+	m0 = 1;
+	m1 = 1;
+	m3 = -1;
+	m4 = 1;
+
+	se = 0;
+	cntr = 256;
+	do _filtering until ce;
+	mx0 = dm(i1,m3);
+	my0 = 0x2000;
+	cntr = 2;
+	mr = mx0*my0(su), mx0 = dm(i0,m3), my0 = pm(i4,m4);
+	do _quads until ce;
+	mr = mr+mx0*my0(ss), mx1 = dm(i0,m3), my0 = pm(i4,m4);
+	mr = mr+mx1*my0(ss), my0 = pm(i4,m4);
+	sr = ashift mr1 (hi), my1 = pm(i4,m4);
+	mr = mr+mx0*my0(ss), mx0 = dm(i0,m0), my0 = pm(i4,m4);
+_quads:
+	dm(i0,m3) = sr1, mr = mr+mx1*my1(ss);
+_filtering:
+	dm(i2,m3) = mr1;
+	rts (db);
+	nop;
+	nop;
+
+_halt:
+	jump _halt;