Extend the ESP32 lwip_select() replacement (direct rcvevent reads +
FreeRTOS task notifications) to all LibreTiny platforms (bk72xx,
rtl87xx, ln882h).
All LibreTiny platforms have LwIP >= 2.1.3 with
lwip_socket_dbg_get_socket() and FreeRTOS task notifications. The
thread safety argument is actually stronger on LibreTiny since all
platforms are single-core ARM Cortex-M, eliminating cross-core
memory ordering concerns entirely.
Introduces USE_LWIP_FAST_SELECT feature define (set from Python
codegen for ESP32 and LibreTiny) replacing per-platform USE_ESP32
guards. The only platform-specific difference is FreeRTOS header
paths (freertos/FreeRTOS.h on ESP-IDF vs FreeRTOS.h on LibreTiny).
Expected impact on LibreTiny (same as ESP32):
- ~17x faster socket polling (direct rcvevent vs lwip_select)
- ~3.5 KB flash savings (dead code elimination of lwip_select)
- ~56 bytes static RAM savings (fd_set members excluded)
- ~200-300 bytes heap savings (UDP wake socket eliminated)
