Fix TypeScript errors under exactOptionalPropertyTypes

Use a local PdfJsXmpMetadata interface instead of the non-exported pdfjs
Metadata type, omit undefined optional fields in reconcileFingerprint, and
type discovery hook mocks with explicit DiscoveryCandidate return types.

src/recovery/reconcile.ts

@@ -20,7 +20,7 @@ export function reconcileFingerprint(
 
   return {
     unchanged,
-    previousFingerprint,
+    ...(previousFingerprint !== undefined ? { previousFingerprint } : {}),
     currentFingerprint,
     requiresReanchor: !unchanged,
   };

src/recovery/recovery.test.ts

@@ -10,6 +10,7 @@ import {
   reIngestAndCompare,
   recoveryStateAfterLocalSearch,
   searchCanonicalQuote,
+  type DiscoveryCandidate,
 } from "./index";
 
 describe("reconcileFingerprint", () => {
@@ -77,15 +78,19 @@ describe("createSourceDiscoveryRegistry", () => {
     const registry = createSourceDiscoveryRegistry();
     registry.register({
       name: "low",
-      discover: vi.fn(async () => [
-        { uri: "file:///a", confidence: 0.2, source: "local" },
-      ]),
+      discover: vi.fn(
+        async (): Promise<readonly DiscoveryCandidate[]> => [
+          { uri: "file:///a", confidence: 0.2, source: "local" },
+        ],
+      ),
     });
     registry.register({
       name: "high",
-      discover: vi.fn(async () => [
-        { uri: "https://example.com/doc", confidence: 0.9, source: "external" },
-      ]),
+      discover: vi.fn(
+        async (): Promise<readonly DiscoveryCandidate[]> => [
+          { uri: "https://example.com/doc", confidence: 0.9, source: "external" },
+        ],
+      ),
     });
 
     const candidates = await registry.discover({ title: "Example" });