feat(cas): content-addressed artifact store (steps 1–8)

New core:artifacts-store interface + infrastructure/artifacts-cas
implementation: segment files + SQLite index, Blake3 hashing,
group-commit fsync via flushBefore, recovery tail-scan, manual
compactor, and oldest-first disk-cap evictor.

Inference events now carry promptArtifactId / responseArtifactId;
orchestrators put bytes before emitting. SqliteEventStore wraps
its txn in artifactStore.flushBefore so segment data is fsynced
before the event commit, making the crash window non-corrupting
(TailScanner re-indexes orphan tail records on reopen).

Compactor and evictor are mutually exclusive via maintenanceMutex.
Step 9 (cloud sync) deferred to a later epic.

See docs/reviews/2026-05-18-cas-steps-1-8-review.md for the final
review.

testing/replay/src/test/kotlin/SessionReplayTest.kt

@@ -23,7 +23,7 @@ class SessionReplayTest {
     private val replayer = DefaultEventReplayer(store, projector)
 
     @Test
-    fun `rebuild session from event stream`() {
+    fun `rebuild session from event stream`(): Unit = kotlinx.coroutines.runBlocking {
         val sessionId = SessionId("s1")
 
         val metadataToPayload = mapOf<EventMetadata, EventPayload>(