fix(client): make ReconnectingOutbox KMP-native-safe (@Volatile→AtomicReference, Math.pow→kotlin.math.pow)

CI red on tag 10 release #1983: client:compileCommonMainKotlinMetadata
and :compileKotlinLinuxArm64 both failed with:
  e: ReconnectingOutbox.kt:154 Unresolved reference 'Volatile'
  e: ReconnectingOutbox.kt:227 Unresolved reference 'Math'

@Volatile is JVM-only annotation; java.lang.Math is JVM-only API. On
linuxArm64/macosArm64 they don't resolve.

Fix:
- @Volatile private var lastSeen: Instant? → AtomicReference<Instant?>
  (kotlin.concurrent.atomics, same module as the AtomicBoolean already
  used for ). .load() / .store() / @OptIn(ExperimentalAtomicApi::class).
- Math.pow(m, e) → m.pow(e) via kotlin.math.pow import.

commonMain stays KMP-clean; jvmTest green (95 tasks); linuxX64 / linuxArm64
/ mingwX64 / macosX64 / macosArm64 compile green.

client/src/commonMain/kotlin/pw/binom/agentik/client/ReconnectingOutbox.kt

@@ -13,8 +13,10 @@ import kotlinx.coroutines.launch
 import pw.binom.agentik.outbox.CommonEvent
 import pw.binom.agentik.outbox.OutboxStore
 import kotlin.concurrent.atomics.AtomicBoolean
+import kotlin.concurrent.atomics.AtomicReference
 import kotlin.concurrent.atomics.ExperimentalAtomicApi
 import kotlin.math.min
+import kotlin.math.pow
 import kotlin.random.Random
 import kotlin.time.Duration
 import kotlin.time.Duration.Companion.seconds
@@ -151,8 +153,8 @@ class ReconnectingOutbox(
     private val started = AtomicBoolean(false)
     private var job: Job? = null
 
-    @Volatile
+    @OptIn(ExperimentalAtomicApi::class)
-    private var lastSeen: Instant? = null
+    private val lastSeen: AtomicReference<Instant?> = AtomicReference(null)
 
     /**
      * Live-события из [outbox] с авто-reconnect. [after] — начальный курсор;
@@ -183,10 +185,11 @@ class ReconnectingOutbox(
     @OptIn(ExperimentalAtomicApi::class)
     private fun ensureStarted(initialCursor: Instant?) {
         if (!started.compareAndSet(false, true)) return
-        lastSeen = initialCursor
+        lastSeen.store(initialCursor)
         job = scope.launch { runLoop() }
     }
 
+    @OptIn(ExperimentalAtomicApi::class)
     private suspend fun runLoop() {
         var attempt = 0
         var connected = false
@@ -194,12 +197,12 @@ class ReconnectingOutbox(
             attempt++
             _status.emit(ConnectionStatus.Connecting(attempt))
             val error: Throwable? = try {
-                outbox.events(after = lastSeen).collect { event ->
+                outbox.events(after = lastSeen.load()).collect { event ->
-                    lastSeen = event.date
+                    lastSeen.store(event.date)
                     _events.emit(event)
                     if (!connected) {
                         connected = true
-                        _status.emit(ConnectionStatus.Connected(lastSeen!!))
+                        _status.emit(ConnectionStatus.Connected(event.date))
                     }
                 }
                 null
@@ -224,7 +227,7 @@ class ReconnectingOutbox(
     private fun computeBackoff(attempt: Int): Duration {
         // attempt 1 → initial, 2 → initial * m, 3 → initial * m^2, ...
         val base = (policy.initial.inWholeMilliseconds.toDouble() *
-                Math.pow(policy.multiplier, (attempt - 1).toDouble()))
+                policy.multiplier.pow((attempt - 1).toDouble()))
             .toLong()
         val capped = min(base, policy.max.inWholeMilliseconds)
         val jitterMs = (capped * policy.jitter * random.nextDouble()).toLong()