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.js

@@ -83,7 +83,9 @@ function fetchStp() {
       // (remote-controlled), never render it as HTML
       if (!stpRows)
         buildPortsTable(s.ports);
-      document.getElementById("stpStat").textContent = s.on
+      document.getElementById("stpStat").textContent = s.fsT
+        ? "\u26a0 STP was disabled by the management failsafe (ports were blocked while management was unreachable). Review the topology before re-enabling."
+        : s.on
         ? (s.weRoot
             ? "This switch is the root bridge (priority 0x" + s.rootPrio + ") — topology changes: " + parseInt(s.tc, 16)
             : "Root bridge: 0x" + s.rootPrio + " / " + s.rootMac
@@ -107,6 +109,7 @@ function fetchStp() {
       document.getElementById("bMaxage").value = s.maxage;
       document.getElementById("bFwd").value = s.fwd;
       document.getElementById("bTxhold").value = s.txhold;
+      document.getElementById("bFailsafe").value = s.fs;
       for (const p of s.ports) {
         document.getElementById("en_" + p.p).value = (p.f & PF_ENABLED) ? "on" : "off";
         document.getElementById("edge_" + p.p).value =
@@ -147,6 +150,8 @@ window.addEventListener("load", function() {
     .addEventListener("change", e => stpCmd("stp fwd " + e.target.value));
   document.getElementById("bTxhold")
     .addEventListener("change", e => stpCmd("stp txhold " + e.target.value));
+  document.getElementById("bFailsafe")
+    .addEventListener("change", e => stpCmd("stp failsafe " + e.target.value));
   document.getElementById("stpMode")
     .addEventListener("change", () => { stpDirty = true; });