Fix heap exhaustion: enable PSRAM allocator + bound all tables + auto-reboot watchdog

Root cause: heltec_V4_boundary build was missing -DRNS_USE_TLSF=1 and
-DRNS_USE_ALLOCATOR=1 flags, causing ALL C++ new/delete to use internal
SRAM (239KB) instead of the PSRAM-backed TLSF pool (~1.6MB). Transport
data structures consumed internal heap until WiFi driver could not
allocate RX buffers (ESP_ERR_NO_MEM).

Changes:
- platformio.ini: Add TLSF/allocator flags to heltec_V4_boundary env,
  re-enable NDEBUG
- Transport.cpp: Add periodic culling of _path_requests (was unbounded,
  grew one entry per unique destination forever). Cull entries older than
  DESTINATION_TIMEOUT. Also cull _pending_local_path_requests for removed
  interfaces, and fix missing .erase() (Python .pop() equivalent).
- RNode_Firmware.ino: Replace WiFi watchdog halt-serial with auto-reboot.
  Add heap pressure check (reboot if free heap < 20KB). Increase WiFi
  grace period from 5s to 15s. Remove orphaned boundary_done label.

platformio.ini

@@ -342,7 +342,9 @@ build_flags =
 	-DARDUINO_USB_CDC_ON_BOOT=1
 	-DBOARD_HAS_PSRAM=1
 	-DBOUNDARY_MODE
-	;-DNDEBUG  ; re-enable debug output for WiFi disconnect investigation
+	-DNDEBUG
+	-DRNS_USE_TLSF=1
+	-DRNS_USE_ALLOCATOR=1
 	; --- Boundary mode defaults (override via EEPROM at runtime) ---
 	; TCP server mode (0=server, 1=client)
 	-DBOUNDARY_TCP_MODE=0