bscan_spi: implement bscan_spi_xfer over the jtagspi proxy

One CS-framed transaction: marker + 32-bit count + MOSI + read-latency
skip + MISO, MSB-first on the wire, matching OpenOCD's jtagspi so the
quartiq proxy bitstreams work unchanged. Half-duplex (tx,txlen,rx,rxlen)
signature, single-device chain.

NOT yet validated on hardware — protocol follows the OpenOCD reference
but has not been confirmed against a live proxy + flash. Validation
(read JEDEC ID on the KCU105) is the next step.

modules/bscan_spi/bscan_spi.h

@@ -50,10 +50,14 @@ int bscan_load_bitstream(jtag_core *jc, const fpga_target *t,
 int bscan_load_bitstream_file(jtag_core *jc, const fpga_target *t,
                               const char *path);
 
-/* Transfer `nbytes` of SPI data through the BSCAN proxy (USER1 DR).
- * Placeholder: protocol details deferred until an actual proxy
- * bitstream is available for testing. */
+/* One CS-framed SPI transaction through the BSCAN proxy (USER1 DR):
+ * clock out `txlen` MOSI bytes (e.g. command + address + write data),
+ * then read `rxlen` MISO bytes into `rx`. Either length may be 0.
+ * Bytes are MSB-first on the wire; bit-order juggling is internal.
+ * Follows the quartiq/OpenOCD jtagspi proxy framing. Single-device
+ * chain only. Requires a proxy bitstream already loaded (USER1 live). */
 int bscan_spi_xfer(jtag_core *jc, const fpga_target *t,
-                   const uint8_t *tx, uint8_t *rx, size_t nbytes);
+                   const uint8_t *tx, size_t txlen,
+                   uint8_t *rx, size_t rxlen);
 
 #endif