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/link.ldf

@@ -0,0 +1,9 @@
+MEMORY
+{
+    int_pm { TYPE(PM RAM) START(0x000000) LENGTH(0x004000) WIDTH(24) }
+    int_dm { TYPE(DM RAM) START(0x000000) LENGTH(0x004000) WIDTH(16) }
+}
+SECTIONS
+{
+    program0 { INPUT_SECTIONS( $OBJECTS(program0) ) } > int_pm
+}