KernelDispatch / KernelRegistry are not safe for concurrent reads #1259

Summary

KernelDispatch (skainet-backend-api) keeps its kernel table in a plain MutableList and guards auto-install with an unsynchronized var autoInstallAttempted; KernelRegistry does the same with its provider list. KernelDispatch.matmul calls ensureInstalled() and find(key) on every dispatch, so two threads dispatching at once (e.g. per-head attention tasks, or two forwards on two contexts) can race: ConcurrentModificationException from ArrayList, a double auto-install, or a lost registration.

Where

• skainet-backends/skainet-backend-api/src/commonMain/kotlin/sk/ainet/backend/api/kernel/KernelDispatch.kt — kernels: MutableList, ensureInstalled(), register(), var defaultSink
• .../KernelRegistry.kt — providers: MutableList, register() (its KDoc already says "not thread-safe")

Fix (in feature/skeep-005-schedules)

Immutable snapshots replaced wholesale under a lock (@Volatile list + a @OptionalExpectation JvmSynchronized that actualizes to kotlin.jvm.Synchronized on JVM/Android and is a no-op elsewhere). Reads stay lock-free. Covered by KernelDispatchConcurrencyTest (16 threads registering/finding; contended ensureInstalled() runs once).

Context

Prerequisite for SKEEP-005 (schedules / structured concurrency): schedule workers dispatch concurrently.