fix(w5100): clear stale Sn_IR bits when INT_N asserts with no RX data

RX_RSR_CHK returned to IDLE when RSR==0 without clearing Sn_IR.  SEND_OK
(bit 4) fires after every successful transmit and the RX path only ever
cleared RECV (bit 2), so INT_N stayed asserted; because IDLE tests
~w5100_int_n BEFORE tx_valid, the FSM span IDLE->RX_CHECK->RX_RSR_CHK->IDLE
and TX was starved permanently.  Observed on hardware as the first frame
transmitting and every later one never setting TI while RX kept working.

Now clears every Sn_IR bit except RECV, so a frame arriving between the RSR
read and the write still raises INT_N and is served next pass.

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

exi_bba/w5100_parallel_master.py

@@ -751,10 +751,23 @@ class W5100ParallelMaster(Elaboratable):
                     m.next = "RX_RSR_CHK"
             with m.State("RX_RSR_CHK"):
                 with m.If(rx_rsr == 0):
-                    m.next = "IDLE"             # nothing received
+                    # INT_N is asserted but there is no data.  Something other
+                    # than RECV set Sn_IR -- in practice SEND_OK (bit 4), which
+                    # fires after every successful transmit.  Returning to IDLE
+                    # without clearing it leaves INT_N low forever, and since
+                    # IDLE tests ~w5100_int_n BEFORE tx_valid, the FSM spins
+                    # IDLE->RX_CHECK->here and TX is starved permanently.  That
+                    # showed up on hardware as: first frame transmits, every
+                    # later one never sets TI, while RX keeps working.
+                    m.next = "RX_IR_CLR"
                 with m.Else():
                     m.next = "RX_RDPTR"
 
+            # Clear every Sn_IR bit EXCEPT RECV (0x04).  Keeping RECV means a
+            # frame that lands between the RSR read and this write still raises
+            # INT_N and gets served on the next pass instead of being lost.
+            write_reg("RX_IR_CLR", _S0_IR, [0x1B], "IDLE")
+
             with m.State("RX_RDPTR"):           # read S0_RX_RD (2 bytes)
                 m.d.sync += [xfer_addr.eq(_S0_RX_RD), xfer_rw.eq(0),
                              xfer_stream.eq(0), xfer_sread.eq(0), xfer_wrap.eq(0),