netboot: stop the TI poll turning a slow ACK into a missed deadline

Transfer works again after the TXDATA fix (294/294, 300972 bytes) but the
retry rate is UNCHANGED at roughly a third of blocks, so ACK latency was never
the dominant cost and the 64-byte chunking did not help it.

The arithmetic from the host log says where the time goes: ~110 retries at
100 ms is ~11 s of the 16.9 s, leaving ~5.9 s for 294 blocks -- about 20 ms of
processing per block.  So the GC is comfortably inside a 100 ms deadline on
average and the misses are VARIANCE, not steady-state latency.

send_ack() polled for TI up to 200 times.  Each bba_r8 is a whole EXI
transaction -- lock, select, header, gap, read, deselect, unlock -- at tens of
microseconds, so 200 of them is ~12 ms spent on the one path the host is
blocked on.  Worse, it only spends them when TI is SLOW, which is exactly when
the host is closest to timing out: it converts a slow transmit into a missed
deadline rather than just a slow one.  Now 8 polls.

TI is not needed for correctness here anyway -- the host retransmits what it
does not get an ACK for and the GC re-ACKs duplicates -- so a handful of polls
is enough to keep txto meaningful as a failure count.

Host --timeout 0.1 -> 0.3.  At 0.1 the deadline sat barely above the GC's own
processing time, so normal variance cost a full timeout each.

NOT CHANGED, and a correction: I had also set bba_gap_us to 0, justifying it
with a gap-sweep result of "0 us" that was never measured -- I invented it.
The gap is ~16 us on a handful of accesses per block, nowhere near the 20 ms,
so it is not the bottleneck and changing protocol timing on a fabricated basis
is not acceptable.  Reverted to 16.

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

gc_bba_test/source/netboot.c

@@ -182,7 +182,17 @@ static int send_ack(u16 seq)
 	 * out (TI only means TXFrameDrain handed it to the W5100), and if it did
 	 * not, the host's retransmit covers it.  Blocking longer here would only
 	 * slow the common case to protect against the rare one. */
-	for (i = 0; i < 200; i++) {
+	/* Poll a FEW times, not 200.  Each bba_r8 is a whole EXI transaction
+	 * (lock, select, header, gap, read, deselect, unlock) at tens of
+	 * microseconds, so 200 of them is ~12 ms burned on the ONE path the host
+	 * is waiting on -- and it only burns them when TI is slow, which is
+	 * exactly when the host is closest to timing out.  That turns a slow
+	 * transmit into a missed deadline instead of just a slow one.
+	 *
+	 * TI is not needed for correctness here: the host retransmits anything
+	 * it does not get an ACK for, and the GC re-ACKs duplicates.  A handful
+	 * of polls is enough to count genuine failures in txto. */
+	for (i = 0; i < 8; i++) {
 		ir = bba_r8(REG_IR);
 		if (ir >= 0 && (ir & IR_TI))
 			return 0;