Avoid intermediate conversation history copies

docs/internal/runner.md

@@ -32,7 +32,8 @@ The runner has no durable run or session store.
 
 Captured exchanges are immutable parent-linked domain values. Each holds only
 its new rendered suffix, final assistant message, and run identities; flattening
-iteratively makes a detached history for preparation or inspection. Preparation
+iteratively fills a single detached message buffer for preparation or projects
+directly into public inspection values. Preparation
 prepends this history to newly rendered definition messages and appended caller
 messages before computing the rendered hash. Follow-up selection and credentials
 remain those of the new request. Prepared execution retains capture and parent

docs/roadmap/remediation.md

@@ -91,7 +91,12 @@ does not require changing runtime behavior or duplicating existing tests.
 
 ## 2. Avoid Intermediate History Copies During Assembly And Inspection
 
-**Priority:** P3. **Status:** For consideration. **Plan stages:** 2 and 5.
+**Priority:** P3. **Status:** Complete. **Plan stages:** 2 and 5.
+
+**Completion:** Preparation now fills one history-plus-suffix buffer. Inspection
+projects indexed history directly into public messages, cloning exposed cache
+metadata once. The parent-linked representation and public types are unchanged.
+Focused race-enabled history, continuation, preparation, and ownership tests passed.
 
 **Locations:** `completePreparationWithStructuredOutput` in
 [the runner](../../internal/usecase/runner.go), `Exchange.Messages` in