feat(core): add SurfEventBus for cross-plugin events

[Copilot]

Introduces a central, platform-independent SurfEventBus so surf-* plugins (surf-playtime, surf-clan, …) can define and fire their own events and listen to others' events without taking a direct dependency on the firing plugin.

Core API (surf-api-core, dev.slne.surf.api.core.event)

• SurfEvent parent + SurfSyncEvent / SurfAsyncEvent base classes and SurfCancellableEvent mixin (with isCancelled / cancel()).
• SurfEventPriority enum (LOWEST…MONITOR) and @SurfEventHandler(priority, ignoreCancelled) annotation.
• SurfEventBus interface with companion object : SurfEventBus by requiredService(), plus registerListeners(vararg), unregisterListeners(vararg), and inline on<T> { … } / onAsync<T> { … } DSL helpers that return a token usable for unregistration.

Invoker infrastructure (dev.slne.surf.api.core.event.invoker, @InternalInvokerApi)

• Java SurfSyncEventInvoker and SurfAsyncEventInvoker functional interfaces.
• Hidden-class templates (SurfSyncEventInvokerTemplate, SurfAsyncEventInvokerTemplate) that reuse the existing InvokerFactory / HiddenInvokerUtil (sync via loadClassData, async via loadClassDataWithAutoSuspend, branching on DATA.isSuspend()).

Implementation (surf-api-core-server)

• SurfEventBusImpl registered via @AutoService(SurfEventBus::class) and guarded with checkInstantiationByServiceLoader().
• Validates handler signatures at registration: exactly one SurfEvent-subtype parameter; suspend rejected for SurfSyncEvent handlers.
• Storage: ConcurrentHashMap<Class, ConcurrentSkipListMap<Priority, CopyOnWriteArrayList<RegisteredHandler>>>. Dispatch matches every registered type assignable from the concrete event class, sorts the union by priority, honours ignoreCancelled, and isolates handler exceptions via Flogger.
• Async dispatch awaits each handler sequentially through suspendCoroutineUninterceptedOrReturn, supporting both suspend and non-suspend handlers transparently.

Sample events (surf-api-paper, dev.slne.surf.api.paper.event.surf)

• AbstractSurfPlayerEvent(playerUuid) async base.
• PlayerAfkStateChangeEvent(playerUuid, fromState, toState) implementing SurfCancellableEvent.

Usage

class AfkListener {
    @SurfEventHandler(priority = SurfEventPriority.HIGH, ignoreCancelled = true)
    suspend fun onAfk(event: PlayerAfkStateChangeEvent) { /* … */ }
}

SurfEventBus.registerListeners(AfkListener())

val token = SurfEventBus.onAsync<PlayerAfkStateChangeEvent> { event -> /* … */ }
SurfEventBus.callAsync(PlayerAfkStateChangeEvent(uuid, false, true))
SurfEventBus.unregisterListeners(token)

Kotlin ABI dumps for surf-api-core and surf-api-paper are regenerated. The Bukkit/Velocity event systems are untouched and coexist with this bus.