feat(acr): ACR Store 1 — content-hash observation cache for repo-file descriptors (#305)

Adds ObservationStore (core:context) keyed on (repoRoot, path), an in-memory default and
a durable SqliteObservationStore (infrastructure:persistence), and wires it into the two
SessionOrchestratorArtifacts call sites that re-derive a SourceDescriptor from CAS bytes
on every call — a hit on matching content hash skips both the CAS read and the regex
extraction. First slice of docs/plans/2026-07-21-acr-knowledge-accretion.md (build order:
observations before fixes/plan-shapes).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HMbPmZZjcXhR2crU82zZ8S

apps/server/src/main/kotlin/com/correx/apps/server/Main.kt

@@ -390,6 +390,7 @@ fun main() {
         semanticReviewer = SemanticReviewerImpl(inferenceRouter),
     )
     val decisionJournalRepository = InfrastructureModule.createDecisionJournalRepository(eventStore)
+    val observationStore = InfrastructureModule.createObservationStore()
     val defaultOrchestrationConfig = OrchestrationConfig(
         sandboxRoot = sandboxRoot,
         defaultSystemPromptPath = toolsConfig.defaultSystemPromptPath,
@@ -496,6 +497,7 @@ fun main() {
         compactionService = journalCompactionService,
         artifactKindRegistry = artifactKindRegistry,
         repoKnowledgeRetriever = repoKnowledgeRetriever,
+        observationStore = observationStore,
         readyTaskCounter = com.correx.apps.server.tasks.ProjectReadyTaskCounter(taskService),
         taskClaimCoordinator = com.correx.apps.server.tasks.DefaultTaskClaimCoordinator(
             taskService,