Watchdog tracks decoded frames, not raw bytes; buffer-overflow backstop (v0.21.2)

Second production zombie slipped past v0.21.1's watchdog: text+telemetry RX
dead for >1 h while TX worked and the watchdog never fired. Root cause proven
at runtime via /proc/net/snmp — InSegs delta = 2 segments per 30 s (OutSegs 0):
the bridge peer keeps dribbling small NON-frame bytes (~every 15 s, keepalive
noise or a desynced stream) and v0.21.1 updated lastPacketAt on EVERY socket
chunk, so 'no traffic for MESH_IDLE_TIMEOUT_MS' was never true even though zero
valid Meshtastic frames decoded.

- The 'data' handler no longer updates lastPacketAt per chunk; it is updated
  only when extractFrames yields >=1 valid packet (the reset in the 'connect'
  handler is unchanged), so a stream that decodes nothing trips the watchdog
  after MESH_IDLE_TIMEOUT_MS even while raw bytes keep arriving.
- Buffer-overflow backstop: if the undecoded frame buffer exceeds
  MESH_MAX_FRAME_BUFFER (new env, default 256000) — e.g. one data event of a
  desynced stream that never forms frames — log ERROR and destroy the socket;
  the existing close -> scheduleReconnect loop self-heals it. The undecoded
  buffer is also reset on every (re)connect so leftover garbage from a dead
  session can't re-trigger the backstop or corrupt the first frames of a
  healthy new stream.

Tests: 169 -> 171 — raw non-frame bytes do NOT keep the watchdog alive
(garbage-only stream forces disconnect + reconnect, today's incident shape),
and an undecoded chunk over MESH_MAX_FRAME_BUFFER triggers destroy + a fresh
session that decodes normally (cap env-overridden to 100 since Node delivers
socket data in ~64 KB events, far below the 256_000 default).

package.json

@@ -1,6 +1,6 @@
 {
   "name": "mesh-bot",
-  "version": "0.21.1",
+  "version": "0.21.2",
   "description": "",
   "author": "",
   "private": true,