feat(confidence-engine): v0.60c identity-aware investigation storage

- loadInvestigation(id) selects by durable ID when provided, null for unknown
- saveInvestigation(snapshot, id) persists under provider-chosen key derived from id
- clearInvestigation(id) removes specific investigation by identity when provided
- localStorage representation: confidence-engine-investigation:<durable-id>
- Backward-compatible singleton path preserved for existing unmigrated callers
- 6 new targeted tests proving two independently addressable Investigations

lib/storage/providers/local-storage.js

@@ -4,6 +4,9 @@ const CANONICAL_KEY  = "confidence-engine-investigation";
 const LEGACY_KEY     = "confidence-engine-session";
 const SCHEMA_VERSION = 1;
 
+// Multi-Investigation key prefix (v0.60c)
+const INVESTIGATION_PREFIX = "confidence-engine-investigation:";
+
 // ── helpers ──────────────────────────────────────────────────────────
 
 function safeGet(storage, key) {
@@ -20,16 +23,34 @@ function isPlainObject(value) {
   );
 }
 
-// ── loadInvestigation ────────────────────────────────────────────────
+// ── loadInvestigation (identity-aware) ───────────────────────────────
 
 /**
- * Returns the persisted investigation snapshot (normalised to current schema)
- * or null when no usable state exists.
+ * Returns the persisted investigation snapshot keyed by durable id
+ * (normalised to current schema), or null when no usable state exists.
+ *
+ * When called without an id argument, reads from the legacy singleton key
+ * for backward-compatible consumers that have not yet migrated.
  */
-export function loadInvestigation() {
+export function loadInvestigation(id) {
   const storage = _getTargetStorage();
   if (!storage) return null;
 
+  // — identity-aware contract: select by durable id —
+  if (id != null) {
+    const raw = safeGet(storage, `${INVESTIGATION_PREFIX}${id}`);
+    if (raw === null) return null;
+    try {
+      const parsed = JSON.parse(raw);
+      if (isPlainObject(parsed)) {
+        if (!("schemaVersion" in parsed)) parsed.schemaVersion = SCHEMA_VERSION;
+        return parsed;
+      }
+    } catch (_) { return null; }
+    return null;
+  }
+
+  // — backward-compatible singleton path (no id supplied) —
   // 1 – canonical key first
   let raw = safeGet(storage, CANONICAL_KEY);
   if (raw !== null) {
@@ -64,36 +85,45 @@ export function loadInvestigation() {
   return parsed;
 }
 
-// ── saveInvestigation ────────────────────────────────────────────────
+// ── saveInvestigation (identity-aware) ───────────────────────────────
 
 /**
- * Persists the supplied snapshot to the canonical localStorage key.
+ * Persists the supplied snapshot under its durable id key.
  * The caller's object is never mutated.
+ *
+ * When called without an id argument, writes to the legacy singleton key
+ * for backward-compatible consumers that have not yet migrated.
  */
-export function saveInvestigation(snapshot) {
+export function saveInvestigation(snapshot, id) {
   const storage = _getTargetStorage();
   if (!storage) return; // silently no-op in non-browser
 
   try {
     const record = JSON.parse(JSON.stringify(snapshot));
     record.schemaVersion = SCHEMA_VERSION;
-    _persist(storage, CANONICAL_KEY, JSON.stringify(record));
+    const key = id != null ? `${INVESTIGATION_PREFIX}${id}` : CANONICAL_KEY;
+    _persist(storage, key, JSON.stringify(record));
   } catch (_) { /* storage errors must not crash caller */ }
 }
 
-// ── clearInvestigation ───────────────────────────────────────────────
+// ── clearInvestigation (identity-aware) ──────────────────────────────
 
-/** Removes the canonical investigation key and the legacy session key. */
-export function clearInvestigation() {
+/**
+ * Removes a specific investigation by durable id.
+ * When called without an id, clears the legacy singleton keys only.
+ */
+export function clearInvestigation(id) {
   const storage = _getTargetStorage();
   if (!storage) return;
-  try { storage.removeItem(CANONICAL_KEY); } catch (_) {}
-
-  // Also remove the legacy sessionStorage key so it cannot resurrect stale
-  // state after loadInvestigation falls through from missing canonical key.
-  const legacyStorage = _getLegacyStorage();
-  if (!legacyStorage) return;
-  try { legacyStorage.removeItem(LEGACY_KEY); } catch (_) {}
+  if (id != null) {
+    try { storage.removeItem(`${INVESTIGATION_PREFIX}${id}`); } catch (_) {}
+  } else {
+    try { storage.removeItem(CANONICAL_KEY); } catch (_) {}
+    const legacyStorage = _getLegacyStorage();
+    if (legacyStorage) {
+      try { legacyStorage.removeItem(LEGACY_KEY); } catch (_) {}
+    }
+  }
 }
 
 // ── internals ────────────────────────────────────────────────────────