stp: vendor-style per-port config + status (path cost, p2p, designated info)

Bring the Spanning Tree page in line with a typical managed switch's
per-port panel. Configuration gains the full-range path cost (raw
0..200000000, 0 = auto, replacing the old 1000x-scaled byte), a
point-to-point admin control (auto/on/off), and the priority is now a
0..240 step-16 dropdown. A new status table shows, per port, the Port
State, Role, Designated Bridge / Port ID / Cost (learned from received
BPDUs, kept per port and aged via the BPDU age), Operational Edge and
Operational Point-to-Point.

The designated fields fall back to presenting this switch as the
segment's designated bridge when no fresh BPDU has been heard (so a
quiet port shows our own bridge-id, as the vendor UIs do). /stp.json
carries the packed hex fields plus our own MAC for that fallback.

Space: reclaim BANK2 for the above by moving rtl837x_pins to HOME and
compacting leds_dump into a register-address table (~800B); bandwidth
returns to BANK1. No BANK3 - hardware-verified that PSBANK > 2 crashes
this SoC at boot (a bricked unit and an SPI-programmer recovery earlier
today); a warning to that effect is now in rtl837x_lldp.c.

Hardware-verified: cost 200000000 and p2p off round-trip through the CLI
and JSON, the status table populates correctly with STP enabled (all
ports Forwarding/Designated, oper-edge and oper-p2p True), LACP 3f/3f
and the LAN unaffected.

(cherry picked from commit 2ec62072f061dc9e78bc821ba1c297cb6819e206)

html/stp.html

@@ -29,10 +29,16 @@
       </tr>
     </table>
     <p style="font-size:small">Changes apply immediately. Edge ports skip the listen period; guard/filter act on received BPDUs.</p>
-    <h2>Ports</h2>
+    <h2>Port configuration</h2>
     <table id="stpPortsTbl">
       <tr>
-        <th>Port</th><th>State</th><th>Role</th><th>STP</th><th>Edge</th><th>Cost [k]</th><th>Priority</th><th>Guard</th><th>Filter</th>
+        <th>Port</th><th>State</th><th>Path Cost<br><span style="font-weight:normal;font-size:small">(0 = Auto)</span></th><th>Priority</th><th>Edge Port</th><th>BPDU Filter</th><th>Guard</th><th>Point-to-Point</th>
+      </tr>
+    </table>
+    <h2>Port status</h2>
+    <table id="stpStatTbl">
+      <tr>
+        <th>Port</th><th>Port State</th><th>Role</th><th>Designated Bridge</th><th>Designated Port ID</th><th>Designated Cost</th><th>Oper. Edge</th><th>Oper. P2P</th>
       </tr>
     </table>
     <script src="/stp.js"></script>