Reduce cast repetition in getLdSignatureObject

Destructure `signature` from `jsonLd` once after the initial type
guard, instead of repeating `jsonLd as { signature?: unknown }` for
each of the three follow-up checks.  Two early returns also replace
the original chained conditions, which reads more naturally.

Keep the return type as `Record<string, unknown> | undefined`.  The
caller in `verifyJsonLd` reads `sig.creator`, `sig.signatureValue`,
and `sig.type` as string properties; switching to `object | undefined`
would push the cast burden into the caller at every property access,
trading one local cast for three remote ones.

#769 (comment)

Assisted-by: Claude Code:claude-opus-4-7

Author: dahlia

packages/fedify/src/sig/ld.ts

@@ -463,14 +463,18 @@ function getLdSignatureObject(
   jsonLd: unknown,
 ): Record<string, unknown> | undefined {
   if (
-    typeof jsonLd === "object" && jsonLd != null && "signature" in jsonLd &&
-    typeof (jsonLd as { signature?: unknown }).signature === "object" &&
-    (jsonLd as { signature?: unknown }).signature != null &&
-    !Array.isArray((jsonLd as { signature?: unknown }).signature)
+    typeof jsonLd !== "object" || jsonLd == null || !("signature" in jsonLd)
   ) {
-    return (jsonLd as { signature: Record<string, unknown> }).signature;
+    return undefined;
   }
-  return undefined;
+  const { signature } = jsonLd as { signature: unknown };
+  if (
+    typeof signature !== "object" || signature == null ||
+    Array.isArray(signature)
+  ) {
+    return undefined;
+  }
+  return signature as Record<string, unknown>;
 }
 
 /**