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

Utilities.h

@@ -807,6 +807,10 @@ int8_t  led_standby_direction = 0;
 #endif
 
 void serial_write(uint8_t byte) {
+	#ifdef BOUNDARY_MODE
+		// No KISS serial output in boundary mode - serial is used for debug logging only
+		return;
+	#endif
 	#if HAS_BLUETOOTH || HAS_BLE == true
 		if (bt_state != BT_STATE_CONNECTED) {
 			#if HAS_WIFI