Boundary mode: performance optimizations + boundary filter

Performance optimizations:
- Move TLSF allocator pool to PSRAM (frees ~170KB internal SRAM)
- Raise TCP_IF_MAX_CLIENTS from 4 to 8 in BOUNDARY_MODE
- Raise path_table_maxsize from 48 to 128, persist from 16 to 32
- Add -DNDEBUG to boundary build: compiles out TRACE/DEBUG macros
- Log level defaults to LOG_VERBOSE when NDEBUG defined
- Serial baud 115200 -> 921600 in BOUNDARY_MODE (reduces CPU blocking)

Previous changes included in this commit:
- Comprehensive boundary filter with transitive whitelisting (7 checks)
- destination_table erase+insert fix (std::map::insert no-overwrite bug)
- Backbone-to-backbone routing guard in next-hop forwarding
- KISS serial output disabled for boundary mode
- flash.py updates for boundary mode support

Config.h

@@ -72,7 +72,11 @@
     #endif
 
 	// MCU independent configuration parameters
+	#ifdef BOUNDARY_MODE
+	const long serial_baudrate  = 921600;
+	#else
 	const long serial_baudrate  = 115200;
+	#endif
 
 	// SX1276 RSSI offset to get dBm value from
 	// packet RSSI register