gc_bba_test: add raw slot dumps to locate the response

Reading one byte and getting 0x00 cannot distinguish 'decode is wrong' from
'reply is late' -- a one-byte window has nowhere for a late answer to show up.
These two probes clock EIGHT data bytes and print every slot, so the position
of 0x17 (NWAYS) or 0x04 (device ID) gives the latency directly, the same way
bit-shift analysis turned 00 00 00 10 into 'correct, 22 bits late'.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

gc_bba_test/source/bba.h

@@ -72,6 +72,10 @@ int bba_write(u16 addr, const void *buf, u32 len);
  * which is a ONE-SHOT -- the first NCRA reset clears it until power cycle. */
 int bba_devid(u8 id[4]);
 
+/* Raw transaction framing, for probes that build their own byte sequence. */
+int  begin_raw(void);
+void end_raw(void);
+
 /* Convenience single-byte helpers.  bba_r8 returns the byte, or <0 on error. */
 int bba_r8(u16 addr);
 int bba_w8(u16 addr, u8 val);