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.

infrastructure/build.gradle

@@ -18,6 +18,8 @@ dependencies {
     implementation project(":infrastructure:tools:filesystem")
     implementation project(":infrastructure:workflow")
     implementation project(":core:artifacts")
+    implementation project(":core:artifacts-store")
+    implementation project(":infrastructure:artifacts-cas")
     implementation "io.ktor:ktor-client-core:$ktor_version"
     implementation "io.ktor:ktor-client-cio:$ktor_version"
     implementation "io.ktor:ktor-client-content-negotiation:$ktor_version"