raspberrypi mdns calls lwip without holding the core lock

[dhalbert]

Claude wrote this:

ports/raspberrypi/common-hal/mdns/Server.c calls lwip's mdns API without holding the lwip core lock.

lwip expects it. LWIP_ASSERT_CORE_LOCKED() is the first statement in mdns_resp_add_service(), mdns_resp_del_service(), mdns_resp_add_netif(), mdns_resp_rename_netif(), mdns_resp_remove_netif() and others (ports/raspberrypi/lib/lwip/src/apps/mdns/mdns.c:2398,2460,2502,2546,2585, among more). It compiles to a no-op here because LWIP_CHECK_CORE_LOCKING isn't set in lwipopts.h, which is presumably why this hasn't been noticed.

The port runs lwip from a low-priority IRQ — async_context_threadsafe_background.c and cyw43_arch_threadsafe_background.c are in ports/raspberrypi/Makefile:43,45 — so these calls really can race with lwip's own processing rather than merely violating a convention.

Unlocked call sites in that file:

• mdns_server_construct() — mdns_resp_init(), mdns_resp_restart(), mdns_resp_add_secondary_hostname()
• common_hal_mdns_server_set_hostname() — mdns_resp_netif_active(), mdns_resp_rename_netif(), mdns_resp_add_netif()
• common_hal_mdns_server_deinit() — mdns_resp_remove_netif()
• common_hal_mdns_server_advertise_service() — mdns_resp_del_service(), mdns_resp_add_service()
• mdns_server_find() and common_hal_mdns_server_find() — mdns_search_service(), mdns_search_stop()

The port already has the primitive: MICROPY_PY_LWIP_ENTER / MICROPY_PY_LWIP_EXIT map to cyw43_arch_lwip_begin() / cyw43_arch_lwip_end() (ports/raspberrypi/mpconfigport.h:50-52), and common-hal/socketpool/ and common-hal/wifi/Radio.c use them.

Care is needed where a locked region can raise — mp_raise_RuntimeError() on a failed mdns_resp_add_service() slot, and m_malloc_fail() — since a longjmp out of a locked region would leave the lock held.

Claude noticed this while reviewing #11181, which locks the TXT record swap in assign_txt_records() but deliberately leaves the rest of the file alone so this can be looked at on its own.