refactor(confidence-engine): v0.60d clarify investigation storage identity

Replace bare re-export in investigation-storage.js with explicit wrapper
functions that own the canonical identity contract: snapshot.id is the sole
save identity authority. The provider never allocates or changes IDs.

7 new deterministic tests prove: identified snapshots persist under their
own id key, explicit competing id arguments are ignored, A/B remain
independently addressable, unknown IDs return null, and legacy singleton
compatibility is preserved for unmigrated callers.

No application callers modified. UI/routes not migrated.

lib/storage/investigation-storage.js

@@ -1,5 +1,34 @@
-// investigation-storage — generic persistence boundary
-// Exposes loadInvestigation / saveInvestigation / clearInvestigation
-// and delegates internally to the concrete localStorage provider.
+// investigation-storage — application-facing persistence boundary
+// Owns the canonical identity contract: snapshot.id is the sole save identity.
+// Delegates to the concrete localStorage provider internally.
 
-export { loadInvestigation, saveInvestigation, clearInvestigation } from "./providers/local-storage.js";
+import { loadInvestigation as _load, saveInvestigation as _save, clearInvestigation as _clear } from "./providers/local-storage.js";
+
+/**
+ * Canonical save contract: snapshot.id is the sole identity authority.
+ * When snapshot carries an id — persist under that id key (identity-aware).
+ * When snapshot has no id — fall back to legacy singleton compatibility path.
+ */
+export function saveInvestigation(snapshot, explicitId) {
+  if (snapshot && typeof snapshot === "object" && snapshot.id != null) {
+    return _save(snapshot, snapshot.id);
+  }
+  // Legacy unidentified snapshot — singleton fallback for unmigrated callers
+  return _save(snapshot, explicitId);
+}
+
+/**
+ * Canonical load contract: select by durable id when supplied;
+ * fall back to legacy singleton path otherwise.
+ */
+export function loadInvestigation(id) {
+  return _load(id != null ? id : undefined);
+}
+
+/**
+ * Canonical clear contract: remove by identity-aware key when supplied;
+ * fall back to legacy singleton keys otherwise.
+ */
+export function clearInvestigation(id) {
+  return _clear(id ?? undefined);
+}