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/integration/build.gradle

@@ -16,8 +16,10 @@ dependencies {
     testImplementation(project(":core:risk"))
     testImplementation(project(":infrastructure:persistence"))
     testImplementation(project(":core:artifacts"))
+    testImplementation(project(":core:artifacts-store"))
     testImplementation(project(":testing:fixtures"))
     testImplementation(project(":testing:kernel"))
     testImplementation(project(":testing:contracts"))
+    testImplementation(testFixtures(project(":testing:contracts")))
     testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:1.9.0")
 }