Support multiple FreeRTOS mDNS instances

[bgewehr]

Problem

Follow-up to #35 and complementary to the per-DeviceLocal event isolation in #44: the FreeRTOS mDNS backend still routes asynchronous query completion through one process-global mdns_inst. With multiple OpenEEBUS services, only the last-created instance receives query completion. Stopping one instance also calls mdns_free(), which tears down the daemon shared by the other instances and the host application.

Change

• use the synchronous mdns_query_ptr() API from each instance's existing browser thread, removing the global callback owner;
• retain browsing and the existing update interval;
• remove only the service entry registered by the stopping instance;
• keep the shared mDNS daemon running;
• initialize and release per-instance thread/semaphore resources; and
• track successful service registration so partial construction and repeated stop remain safe.

No public OpenEEBUS API changes are introduced.

Verification

• clang-format 18: clean
• ESP32-S3 Xtensa / ESP-IDF 5.5.4 target object: compiled successfully
• GCC 14 / Debian Trixie: complete host test suite passed

The target compile used Espressif mDNS 1.11.0, where multiple service instances are enabled by default.