stp: management failsafe (commit-confirm) + bounded NIC waits

Enabling STP on a bridge whose management rides an in-band VLAN can cut
off that very management - and not only by our own blocking: on this
network the upstream TP-Link Easy Smart switch's "loop prevention"
reacted to our BPDU hellos by blocking ITS port towards us while our
ASIC was all-forwarding, isolating the whole segment until a power
cycle. Recoverable only by going quiet.

Add a commit-confirm watchdog: while STP is enabled, any HTTP request
re-arms a countdown ("stp failsafe <seconds>", default 180, 0 disables);
if management stays silent for the whole window, STP disables itself,
which also stops BPDU TX so a neighbour's loop protection can release
its block. The web UI polls /stp.json every 2 s, so an open browser
naturally keeps the watchdog re-armed. The trip is reported via
/stp.json (fs, fsT) and as a warning on the Spanning Tree page.

Deliberately not conditioned on our own MSTP port states - the incident
above proves the uplink can be dead while every local port forwards.

Also bound the NIC DMA busy-waits (nic_tx_packet, nic_rx_header,
nic_rx_packet): an unbounded spin on SFR_NIC_CTRL freezes the entire
main loop (timers, HTTP, ARP) if the ASIC ever fails to consume a
transfer; give up after ~65k polls and drop the frame instead.

Hardware-verified end to end: with priority 15 against a live RSTP
bridge the uplink died 6 s after "stp on" and the network recovered BY
ITSELF 66 s later (trip at 45 s + neighbour release), fsT=1, LACP and
LAN intact. Telemetry via syslog-to-edge-port host confirmed the full
chain: countdown 44->4, trip, hello TX stopping at the trip.

(cherry picked from commit 1fa9775156fd6d7ebfdda2382f73430b86601230)

html/stp.html

@@ -16,7 +16,7 @@
     <h2>Bridge settings</h2>
     <table id="stpBridge">
       <tr>
-        <th>Priority</th><th>Version</th><th>Hello [s]</th><th>Max age [s]</th><th>Fwd delay [s]</th><th>Tx hold</th>
+        <th>Priority</th><th>Version</th><th>Hello [s]</th><th>Max age [s]</th><th>Fwd delay [s]</th><th>Tx hold</th><th>Mgmt failsafe [s]</th>
       </tr>
       <tr>
         <td><select id="bPrio"></select></td>
@@ -25,6 +25,7 @@
         <td><input id="bMaxage" type="number" min="6" max="40" style="width:4em"></td>
         <td><input id="bFwd" type="number" min="4" max="30" style="width:4em"></td>
         <td><input id="bTxhold" type="number" min="1" max="10" style="width:4em"></td>
+        <td><input id="bFailsafe" type="number" min="0" max="255" style="width:4em" title="Auto-disable STP if ports stay blocked while management is silent; 0 = off"></td>
       </tr>
     </table>
     <p style="font-size:small">Changes apply immediately. Edge ports skip the listen period; guard/filter act on received BPDUs.</p>