fix(security): synchronize SecureVault::scope_ between the main thread and worker security ops

Concurrency audit: SecureVault::scope_ (std::string) is written by setWalletScope() on the main thread
(wallet switch) while getVaultPath() — reached from retrieve()/hasVault()/changePin()/store() — runs on
RPC worker threads during a send-time or startup unlock. A concurrent wallet switch and a worker vault op
is a data race on the string (and could resolve the wrong vault file).

Guard scope_ with a small mutex: setWalletScope() assigns under it; getVaultPath() copies it out under the
lock and uses the copy. The lock is held only for the string copy — never across Argon2id or file I/O —
and getVaultPath() never calls setWalletScope(), so no deadlock. Argon2id params, key derivation, AEAD,
the vault file format and retrieve/store/changePin are byte-identical. SecureVault is held via unique_ptr
and used by pointer, so the added (non-movable) mutex is fine.

Synchronization only. Threading fix — clean build + ctest; the race is runtime-only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/util/secure_vault.h

@@ -7,6 +7,7 @@
 #include <string>
 #include <vector>
 #include <cstdint>
+#include <mutex>
 
 namespace dragonx {
 namespace util {
@@ -101,6 +102,9 @@ private:
                    const uint8_t* salt, size_t saltLen,
                    uint8_t* key, size_t keyLen) const;
 
+    // Guards scope_: setWalletScope() runs on the main thread (wallet switch) while retrieve()/hasVault()/
+    // changePin() run on RPC worker threads and read it via getVaultPath().
+    mutable std::mutex scopeMutex_;
     std::string scope_;   // "" = default/legacy wallet (vault.dat); else a sanitized wallet-file tag
 };