stp: full RSTP configuration (bridge + per-port), CLI + GUI + persistence

Implements the standard 802.1D-2004/802.1w configuration surface:

Bridge:  priority (0-15 x4096), hello time, max age, forward delay,
         force-version (RSTP v2 / STP-compatible v0 Config BPDUs), tx hold
         count (per-port per-second BPDU budget).
Per port: enable, admin edge (forwarding immediately - no listen gap),
         auto edge (forwarding after 3 s of BPDU silence; DEFAULT, so
         host-facing ports no longer take the full forward delay),
         path cost (0=auto/20000), port priority, BPDU guard (port disabled
         on BPDU receipt), root guard (never accept a better root on the
         port), BPDU filter (no BPDUs in or out).

Engine additions: root max-age expiry (reclaim the tree when the root goes
silent), root path cost accounting (rx cost + root-port cost, advertised in
our BPDUs), loop detection (our own BPDU coming back blocks the port for a
listen period), topology-change counter, approximated per-port roles
(Root/Designated/Alternate) for diagnostics.

CLI: "stp prio|hello|maxage|fwd|txhold|version ..." and
"stp port <n> on|off|edge|cost|prio|guard|filter ..." (stp_parse, delegated
from cmd_parser); all forms accepted by the startup-config validator so the
whole configuration persists. /stp.json now reports config + status; the
Spanning Tree page exposes everything with immediate-apply controls and live
state/role columns (edit-in-flight guard against the 2 s refresh).

8051 memory: the module moves to code BANK2; internal-RAM pressure from
cross-bank calls resolved by xdata loop iterators/scratch, __reentrant on
the small helpers, and moving httpd's header-pointer globals to xdata.

Verified on hardware (SWTGW218AS): defaults land per standard; priority and
hello change live; admin-edge ports (the LACP bond uplinks) keep the LAN at
0% loss THROUGH "stp on"; auto-edge ports forward after 3 s; a port that
heard real BPDUs (a VM bridge behind physical port 6) correctly declined
auto-edge, sat out the full listen period and became Designated; tc counts
promotions; we win the root election at priority 16384 vs 32768.

(cherry picked from commit 09a34dc6acdc81ab9cab0727d2f4a59c68131a3e)

cmd_parser.c

@@ -1525,15 +1525,7 @@ void cmd_parser(void) __banked
 			else
 				igmp_setup();  // Reverts to default with IP-MC being flooded
 		} else if (cmd_compare(0, "stp")) {
-			if (cmd_compare(1, "on")) {
-				print_string("STP enabled\n");
-				stpEnabled = 1;
-				stp_setup();
-			} else {
-				print_string("STP disabled\n");
-				stp_off();
-				stpEnabled = 0;
-			}
+			stp_parse();
 		} else if (cmd_compare(0, "pvid") && cmd_words_len == 3) {
 			__xdata uint16_t pvid;
 			uint8_t port;