Index stable history trees during normalization

docs/roadmap/remediation.md

@@ -212,7 +212,15 @@ in one focused overlay regression.
 
 ## 4. History Construction Repeatedly Performs Unindexed Mapping Searches
 
-Priority: P2. Status: open.
+Priority: P2. Status: complete.
+
+Completion: checkpoints, inheritance sides, and resolution before/final states
+use one immutable document lookup per stable state, rather than scanning broad
+mappings for every path. Lookups are rebuilt after scratch replay changes.
+Removed the unused summary contribution list and its searches; the existing
+test still verifies composite metadata and individual explanation attribution.
+The full ordinary suite and `git diff --check` passed, including collection,
+inheritance, resolution shape, and typed-checkpoint cases.
 
 Relevant code: `observeTypedCheckpoint` in
 [history_cli.go](../../internal/engine/history_cli.go), lines 38–49;