Rework the EXI decoder to the real GC<->BBA protocol

Replaces the invented 2-byte header (addr split across both bytes, 2-bit
length field) with the protocol libogc and Swiss actually speak:

  command  (bit7=0, 6-bit reg)   [reg, 00, 00, data]  one 4-byte transfer
  register (bit7=1, 16-bit reg)  [0x80|w<<6, ah, al, 00] + data until CS

This was not an optimisation: the old decoder read Swiss's NWAYS read
(80 00 31 00) as 'write 0x31 to NCRA', and bit 0 of 0x31 is RESET, so every
register read would have reset the device.

Changes:
  * 16-bit address; no length field -- register reads and writes now stream
    until CS, like the SPRAM path already did
  * RHDR_LO starts the SPRAM fetch as soon as the address completes, using
    the trailing header byte as the prefetch window the protocol provides
  * command space serves EXI regs 0x02/0x03 (interrupt mask/status), the only
    two Swiss uses
  * EXI_GetID gets its own path: it offers just ONE header byte of warning, so
    the first ID byte is pushed the instant byte 0 identifies the query

Two timing subtleties, both found by simulation and commented in place:
  * MISO is driven LIVE from the tx FIFO head, so a byte pushed while the pad
    byte is on the wire appears in the pad slot.  A filler byte is parked
    there (MISO is don't-care through the header) so the first real response
    lands in data slot 0.
  * the prefetch must be one-shot; re-firing while waiting for the pad byte
    consumed the register stream early and shifted reads by a byte.

All testbenches updated to the real format and passing: bba_register_file
(7), bba_top (8 incl. the full W5100->SPRAM->GC loop and TX), exi_j4,
exi_capture, spi_mode3_slave, tx_frame_drain, spram_arbiter,
rx_frame_assembler, w5100_parallel_master.  gc_bba_test rebuilt against the
new protocol.

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

gc_bba_test/source/tests.c

@@ -19,7 +19,7 @@ static int t_devid(char *m, size_t n)
 		snprintf(m, n, "skipped: one-shot, needs a console power cycle");
 		return 2;
 	}
-	if ((rc = bba_read(0x0000, b, 4)) < 0) { snprintf(m, n, "EXI error %d", rc); return 1; }
+	if ((rc = bba_devid(b)) < 0) { snprintf(m, n, "EXI error %d", rc); return 1; }
 	snprintf(m, n, "%02X %02X %02X %02X (want 04 02 02 00)", b[0], b[1], b[2], b[3]);
 	return (b[0]==0x04 && b[1]==0x02 && b[2]==0x02 && b[3]==0x00) ? 0 : 1;
 }