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.

core/validation/src/main/kotlin/com/correx/core/validation/artifact/ArtifactPayloadValidator.kt

@@ -3,7 +3,6 @@ package com.correx.core.validation.artifact
 import com.correx.core.artifacts.kind.FileWrittenArtifact
 import com.correx.core.artifacts.kind.ProcessResultArtifact
 import com.correx.core.artifacts.kind.TypedArtifactSlot
-import com.correx.core.artifactstore.ArtifactStore
 import com.correx.core.events.types.ArtifactId
 import com.correx.core.validation.model.ValidationContext
 import com.correx.core.validation.model.ValidationIssue
@@ -13,16 +12,14 @@ import com.correx.core.validation.pipeline.Validator
 import kotlinx.serialization.json.Json
 import java.nio.file.Path
 
-class ArtifactPayloadValidator(
-    private val artifactStore: ArtifactStore,
-) : Validator {
+class ArtifactPayloadValidator : Validator {
 
     override suspend fun validate(context: ValidationContext): ValidationSection {
         val issues = mutableListOf<ValidationIssue>()
 
         for ((_, stageConfig) in context.graph.stages) {
             for (slot in stageConfig.produces) {
-                validateSlot(slot, issues)
+                validateSlot(slot, context.producedArtifactContent, issues)
             }
         }
 
@@ -31,11 +28,10 @@ class ArtifactPayloadValidator(
 
     private suspend fun validateSlot(
         slot: TypedArtifactSlot,
+        artifactContent: Map<String, String>,
         issues: MutableList<ValidationIssue>,
     ) {
-        val bytes = artifactStore.get(slot.name) ?: return
-
-        val payloadStr = bytes.toString(Charsets.UTF_8)
+        val payloadStr = artifactContent[slot.name.value] ?: return
 
         when (slot.kind.id) {
             "file_written" ->

core/validation/src/main/kotlin/com/correx/core/validation/model/ValidationContext.kt

@@ -11,4 +11,5 @@ data class ValidationContext(
     val cyclePolicies: Set<CyclePolicyBinding> = emptySet(),
     val sessionState: SessionState? = null,
     val availableTools: Set<String>? = null,
+    val producedArtifactContent: Map<String, String> = emptyMap(),
 )