feat(confidence-engine): v0.60g1 list investigation summaries

Recover to clean v0.60f then implement only the storage listing contract.

- Add listInvestigations() to provider: enumerate by prefix, project lightweight summary (id, scenario, updatedAt, investigationRevision, reportExists, reportGeneratedFromRevision), sort by updatedAt desc
- Add application-facing wrapper in investigation-storage.js
- Add 8 deterministic tests covering all listing invariants (coexistence, correct IDs, lightweight projection, legacy exclusion, unrelated exclusion, independent update, ordering, malformed skip)
- Fix MockStorageMap WebStorage API compatibility (.length + .key(i))
- Portfolio NOT migrated — that is v0.60g2

lib/storage/investigation-storage.js

@@ -2,7 +2,7 @@
 // Owns the canonical identity contract: snapshot.id is the sole save identity.
 // Delegates to the concrete localStorage provider internally.
 
-import { loadInvestigation as _load, saveInvestigation as _save, clearInvestigation as _clear } from "./providers/local-storage.js";
+import { loadInvestigation as _load, saveInvestigation as _save, clearInvestigation as _clear, listInvestigations as _list } from "./providers/local-storage.js";
 
 /**
  * Canonical save contract: snapshot.id is the sole identity authority.
@@ -32,3 +32,11 @@ export function loadInvestigation(id) {
 export function clearInvestigation(id) {
   return _clear(id ?? undefined);
 }
+
+/**
+ * Lists all durable-ID Investigation records as lightweight summaries.
+ * Excludes legacy singleton, sessionStorage state, unrelated storage, malformed entries.
+ */
+export function listInvestigations() {
+  return _list();
+}