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.

core/artifacts-store/build.gradle

@@ -0,0 +1,9 @@
+plugins {
+    id 'java-library'
+    id 'org.jetbrains.kotlin.jvm'
+    id 'org.jetbrains.kotlin.plugin.serialization'
+}
+
+dependencies {
+    implementation(project(":core:events"))
+}

core/artifacts-store/src/main/kotlin/com/correx/core/artifactstore/ArtifactStore.kt

@@ -0,0 +1,9 @@
+package com.correx.core.artifactstore
+
+import com.correx.core.events.types.ArtifactId
+
+interface ArtifactStore {
+    suspend fun put(bytes: ByteArray): ArtifactId
+    suspend fun get(id: ArtifactId): ByteArray?
+    suspend fun flushBefore(commit: suspend () -> Unit)
+}