fix(validation): resolve F-007 artifact validator CAS-hash conflation

ArtifactPayloadValidator passed the logical slot name into the
content-hash-keyed CAS store, crashing every workflow with a typed
produces slot. Validator now reads payloads from a content map threaded
through ValidationContext (populated from the orchestrator's per-session
artifact cache across all stages) and no longer depends on ArtifactStore.

Also records the slot->CAS-hash mapping as a new ArtifactContentStoredEvent
(registered for serialization), emitted at both CAS write sites, so
artifact content is recoverable from CAS by hash after a cold start.

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

@@ -225,7 +225,7 @@ fun main() {
         inferenceRouter = inferenceRouter,
         validationPipeline = ValidationPipeline(
             validators = listOf(
-                ArtifactPayloadValidator(artifactStore),
+                ArtifactPayloadValidator(),
                 SemanticValidator(
                     rules = listOf(
                         CycleExitRule(requirePolicyForCycles = false),