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.

settings.gradle

@@ -21,6 +21,7 @@ include ':core:inference'
 include ':core:stages'
 include ':core:agents'
 include ':core:artifacts'
+include ':core:artifacts-store'
 include ':core:validation'
 include ':core:approvals'
 include ':core:tools'
@@ -41,6 +42,7 @@ include ':infrastructure:security'
 include ':infrastructure:scheduler'
 include ':infrastructure:telemetry'
 include ':infrastructure:workflow'
+include ':infrastructure:artifacts-cas'
 
 include ':interfaces:api'
 include ':interfaces:cli'