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>

gc_bba_test/source/main.c

@@ -125,6 +125,21 @@ static void nb_status(const char *msg)
 	printf("  %s\n", msg);
 }
 
+/* Hold the screen until a button is pressed.  Without this the caller's
+ * draw_main() redraws immediately and wipes every diagnostic line netboot
+ * just printed -- which is exactly why the first two failures were reported
+ * as "a short flash" with no detail. */
+static void nb_pause(const char *why)
+{
+	printf("\n" C_DIM "  %s -- press any button to return to the menu\n" C_RESET,
+	       why);
+	for (;;) {
+		VIDEO_WaitVSync();
+		PAD_ScanPads();
+		if (PAD_ButtonsDown(0)) return;
+	}
+}
+
 static void do_netboot(char *status, size_t nstatus)
 {
 	int len, rc;
@@ -147,11 +162,13 @@ static void do_netboot(char *status, size_t nstatus)
 	len = netboot_receive(s_stage, s_stage_max, nb_status);
 	if (len <= 0) {
 		snprintf(status, nstatus, "net-update failed (%d)", len);
+		nb_pause("transfer failed");
 		return;
 	}
 
 	rc = netboot_sd_write(netboot_dest_path(), s_stage, (u32)len, nb_status);
 	if (rc < 0) {
+		nb_pause("SD write failed");
 		/* Nothing has been launched and nothing is half-written that we
 		 * know of, so staying in the menu is the safe outcome. */
 		snprintf(status, nstatus,
@@ -166,6 +183,7 @@ static void do_netboot(char *status, size_t nstatus)
 	/* Reuse the staging region: the new build is already on the card and
 	 * verified, so the copy in RAM is finished with. */
 	rc = netboot_boot_file("/boot.dol", s_stage, s_stage_max, nb_status);
+	nb_pause("boot refused");
 
 	/* Only reached on refusal.  The new build IS on the card either way,
 	 * so this is recoverable by rebooting into Swiss by hand. */