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)

httpd/httpd.c

@@ -41,8 +41,8 @@ __xdata uint32_t cont_addr;
 
 // HTTP header properties
 __xdata uint8_t boundary[72];
-__xdata uint8_t *content_type = 0;
-__xdata uint8_t *session = 0;
+__xdata uint8_t * __xdata content_type = 0;
+__xdata uint8_t * __xdata session = 0;
 
 // Global variables holding POST state
 __xdata uint16_t bindex; // Current index into the boundary
@@ -54,7 +54,7 @@ __xdata char passwd[21];
 __xdata char session_id[SESSION_ID_LENGTH + 1];
 __xdata uint8_t authenticated;
 __xdata uint32_t now;
-__xdata uint8_t *timeptr;
+__xdata uint8_t * __xdata timeptr;
 __xdata uint32_t last_session_use;
 
 #define TSTATE_NONE		0