netboot: hold the screen on failure, dump the descriptor, shrink the block

Two separate problems, one of which was hiding the other.

1. The diagnostics were unreadable.  do_netboot() printed them and returned,
   and the caller's draw_main() redrew immediately and wiped the screen -- so
   two consecutive failures were reported only as "a short flash" with no
   detail, and the error code alone was not enough to act on.  nb_pause() now
   holds the screen until a button is pressed, on every failure path.

   This cost two blind round trips, each of which needs a hand copy of the
   .dol to the SD card.  Printing a diagnosis and then erasing it is worse
   than not printing one.

2. The default block was 1024 bytes, making ~1050-byte frames that span FIVE
   of the SPRAM ring's 15 pages.  The only multi-page frame ever proven on
   this hardware is the 342-byte DHCP broadcast from bring-up, which spans
   two.  Five-page frames are untested territory, and a transfer is the wrong
   place to discover a wrap bug.

   Default is now 256 bytes -> ~296-byte frame -> two pages, deliberately
   close to the proven case.  --block raises it once small blocks work; the GC
   still caps at NB_BLOCK_MAX = 1024.

ring_next() also keeps the four raw descriptor bytes and the page it read
them from, and the error path prints them.  When a length is rejected those
bytes ARE the diagnosis: 00 00 xx xx tells us the assembler wrote a
descriptor and the length is simply unexpected, whereas garbage tells us we
are reading the wrong page.

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

scripts/send-dol.py

@@ -25,7 +25,13 @@ import time
 ETHERTYPE = 0x88B5
 MAGIC = b"GCUP"
 OP_DATA, OP_ACK, OP_DONE = 1, 2, 3
-BLOCK = 1024
+# Default payload 256 bytes -> a ~296-byte frame, which with the 4-byte
+# descriptor occupies TWO of the SPRAM ring's 15 pages.  That is deliberately
+# close to the only multi-page frame ever proven on this hardware (a 342-byte
+# DHCP broadcast).  1024-byte payloads make ~1050-byte frames spanning FIVE
+# pages, which has never been exercised -- raise --block once small blocks are
+# known to work, not before.
+BLOCK_DEFAULT = 256
 HDR = 26
 # The GC's MAC comes from PAR0-5, which the gateware pre-populates with a
 # Nintendo OUI.  Discovered from the first ACK, so this is only the initial
@@ -54,10 +60,15 @@ def main():
     ap.add_argument("--dest", default="/gc_bba_test.dol",
                     help="where to write it on the SD card "
                          "(default /gc_bba_test.dol)")
+    ap.add_argument("--block", type=int, default=BLOCK_DEFAULT,
+                    help=f"payload bytes per frame (default {BLOCK_DEFAULT}; "
+                         "the GC caps it at 1024)")
     args = ap.parse_args()
 
+    if not 1 <= args.block <= 1024:
+        sys.exit("--block must be 1..1024 (the GC's NB_BLOCK_MAX)")
     img = open(args.dol, "rb").read()
-    blocks = [img[i:i + BLOCK] for i in range(0, len(img), BLOCK)]
+    blocks = [img[i:i + args.block] for i in range(0, len(img), args.block)]
     if not blocks:
         sys.exit("empty image")
     if len(blocks) > 0xFFFF:
@@ -68,7 +79,7 @@ def main():
     src = s.getsockname()[4]
     s.settimeout(args.timeout)
 
-    print(f"{args.dol}: {len(img)} bytes, {len(blocks)} blocks of {BLOCK}")
+    print(f"{args.dol}: {len(img)} bytes, {len(blocks)} blocks of {args.block}")
     print(f"sending from {src.hex(':')} on {args.iface}")
 
     dst = BROADCAST