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/kernel/build.gradle

@@ -14,5 +14,7 @@ dependencies {
     implementation project(':core:inference')
     implementation project(':core:tools')
     implementation project(':core:artifacts')
+    implementation project(':core:artifacts-store')
     implementation project(':core:risk')
+    implementation "org.slf4j:slf4j-api:2.0.16"
 }