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/fixtures/src/main/kotlin/com/correx/testing/fixtures/InferenceFixtures.kt

@@ -4,12 +4,14 @@ import com.correx.core.context.model.ContextPack
 import com.correx.core.events.events.EventMetadata
 import com.correx.core.events.events.InferenceCompletedEvent
 import com.correx.core.events.events.NewEvent
+import com.correx.core.events.types.ArtifactId
 import com.correx.core.events.types.ContextPackId
 import com.correx.core.events.types.EventId
 import com.correx.core.events.types.InferenceRequestId
 import com.correx.core.events.types.ProviderId
 import com.correx.core.events.types.SessionId
 import com.correx.core.events.types.StageId
+import com.correx.core.utils.TypeId
 import com.correx.core.inference.FinishReason
 import com.correx.core.inference.GenerationConfig
 import com.correx.core.inference.InferenceProvider
@@ -73,6 +75,9 @@ object InferenceFixtures {
         )
     }
 
+    private const val PLACEHOLDER_HEX_BYTES = 32
+    private val PLACEHOLDER_ARTIFACT_ID: ArtifactId = TypeId("00".repeat(PLACEHOLDER_HEX_BYTES))
+
     fun inferenceCompleted(
         requestId: InferenceRequestId,
         sessionId: SessionId,
@@ -80,6 +85,7 @@ object InferenceFixtures {
         providerId: ProviderId,
         tokensUsed: TokenUsage,
         latencyMs: Long,
+        responseArtifactId: ArtifactId = PLACEHOLDER_ARTIFACT_ID,
     ): InferenceCompletedEvent {
         return InferenceCompletedEvent(
             requestId = requestId,
@@ -88,6 +94,7 @@ object InferenceFixtures {
             providerId = providerId,
             tokensUsed = tokensUsed,
             latencyMs = latencyMs,
+            responseArtifactId = responseArtifactId,
         )
     }
 
@@ -127,6 +134,7 @@ object InferenceFixtures {
         providerId: ProviderId,
         tokensUsed: TokenUsage,
         latencyMs: Long,
+        responseArtifactId: ArtifactId = PLACEHOLDER_ARTIFACT_ID,
     ): NewEvent {
         return NewEvent(
             metadata = EventMetadata(
@@ -144,6 +152,7 @@ object InferenceFixtures {
                 providerId = providerId,
                 tokensUsed = tokensUsed,
                 latencyMs = latencyMs,
+                responseArtifactId = responseArtifactId,
             ),
         )
     }