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)

rtl837x_pins.c

@@ -2,8 +2,6 @@
 #include "rtl837x_common.h"
 #include "rtl837x_regs.h"
 
-#pragma codeseg BANK2
-#pragma constseg BANK2
 
 uint8_t i2c_bus_from_sda_pin(uint8_t sda_pin) __banked {
 	switch (sda_pin) {