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/contracts/build.gradle

@@ -17,7 +17,9 @@ dependencies {
     testImplementation(project(":core:context"))
     testFixturesImplementation(project(":core:events"))
     testFixturesImplementation(project(":core:sessions"))
+    testFixturesImplementation(project(":core:artifacts-store"))
     testFixturesImplementation(project(":testing:fixtures"))
     testFixturesImplementation "org.jetbrains.kotlinx:kotlinx-datetime:$kotlinx_datetime_version"
     testFixturesImplementation "org.junit.jupiter:junit-jupiter:$junit_version"
+    testFixturesImplementation "org.jetbrains.kotlinx:kotlinx-coroutines-core:$kotlinx_coroutines_version"
 }