Fix sd-bus thread-safety crash in Linux tray (#405)

[kdroidFilter]

Summary

Fixes #405 — Assertion '!bus->current_slot' failed at ... bus_process_internal(). Aborting.

sd-bus objects are not thread-safe. In sni.c the event-loop thread (sni_tray_run → sd_bus_process) and the JVM threads driving every sni_tray_set_* / sni_tray_item_* / sni_tray_add_* mutator (→ sd_bus_emit_*) accessed the same tray->bus with zero synchronization. Under frequent icon/menu updates an emit races with an in-flight sd_bus_process and trips the internal current_slot assertion, aborting the process.

Changes

• Add a bus_lock mutex that serializes all sd_bus_* access and the tray state it reads/writes — held across bus setup, each sd_bus_process batch, and teardown, and acquired by every public mutator.
• Defer user callbacks (click / menu) through a queue that the event loop flushes after releasing bus_lock. This avoids holding the bus lock across an upcall into Kotlin, which would otherwise deadlock against the LinuxTrayManager ReentrantLock (lock inversion when a callback re-enters the mutators).
• Add a standalone C concurrency regression test (test_sni_concurrency.c + run_concurrency_test.sh).

Test plan

• Regression test aborts on the old code (exit 134, exact !bus->current_slot assertion)
• Regression test passes on the fixed code: 16 runs × 200k contended rounds, zero crashes
• build.sh rebuilds libLinuxTray.so clean with -Wall -Wextra (no new warnings)

Run the test: ./src/native/linux/run_concurrency_test.sh (uses a private dbus-run-session).