fix(wallet-switch): close gaps found verifying clusters A/B/C

Adversarial verification of the audit fixes found real gaps in them:

- HIGH throw-safety: the switch worker and the encryption-restart worker set
  daemon_restarting_=true but reset it only on their normal/early-return paths —
  a throw from stop/startEmbeddedDaemon left the flag stuck true, wedging
  reconnect and every future switch/rescan/encryption. Both now reset it on all
  paths (try/catch); a throw during a switch is treated as a failed switch and
  triggers the revert.
- HIGH residual chat leak: resetChatSession() cleared the flags but an already-
  posted z_exportmnemonic worker job still held wallet A's secret, and its
  completion callback (guarded only by isLocked(), false for an unencrypted
  wallet) would provision A's identity under B. Add a chat_session_generation_
  epoch bumped on every wallet change; the fetch captures it and its callback
  discards the (previous-wallet) secret if the epoch no longer matches.
- LOW vault-scope collision: the per-wallet vault tag was a lossy char-substitution
  (two distinct files could map to one vault). Append an 8-hex FNV-1a of the raw
  filename so distinct wallets never share a vault. +unit test.
- LOW seed-adopt: removeVault() on adopt so the legacy wallet's PIN passphrase
  isn't left associated with the new seed wallet (same file name).
- LOW: clear lock_unlock_in_progress_ on switch too.

Deferred (documented): the 1.5s start grace can't catch a wallet that fails LATE
in daemon init (the existing crash-wedge detection still applies); beginShutdown's
join of the switch task can briefly freeze the UI during quit (necessary to avoid
orphaning the daemon).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_phase4.cpp

@@ -6,6 +6,7 @@
 #include "data/transaction_history_cache.h"
 #include "data/address_book.h"
 #include "data/wallet_index.h"
+#include "util/secure_vault.h"
 #include "daemon/lifecycle_adapters.h"
 #include "data/wallet_state.h"
 #include "rpc/connection.h"
@@ -2139,6 +2140,34 @@ void testOperationStatusPollParsing()
     EXPECT_TRUE(malformed.staleOpids.empty());
 }
 
+void testSecureVaultScope()
+{
+    using dragonx::util::SecureVault;
+    auto endsWith = [](const std::string& s, const std::string& suf) {
+        return s.size() >= suf.size() && s.compare(s.size() - suf.size(), suf.size(), suf) == 0;
+    };
+
+    // Default wallet (empty or "wallet.dat") keeps the legacy global vault.dat for back-compat.
+    EXPECT_TRUE(endsWith(SecureVault("").getVaultPath(), "vault.dat"));
+    EXPECT_TRUE(endsWith(SecureVault("wallet.dat").getVaultPath(), "vault.dat"));
+
+    // A non-default wallet gets its own, distinct vault (not the legacy path).
+    SecureVault vb("wallet-savings.dat");
+    const std::string pb = vb.getVaultPath();
+    EXPECT_TRUE(pb.find("vault-") != std::string::npos);
+    EXPECT_TRUE(pb != SecureVault("").getVaultPath());
+    EXPECT_TRUE(pb != SecureVault("wallet-cold.dat").getVaultPath());
+
+    // The FNV suffix disambiguates sanitize-colliding names, so distinct files never share a vault.
+    EXPECT_TRUE(SecureVault("wallet foo.dat").getVaultPath() != SecureVault("wallet_foo.dat").getVaultPath());
+
+    // Re-scoping moves the path; scoping back to the default returns to vault.dat.
+    vb.setWalletScope("wallet-cold.dat");
+    EXPECT_TRUE(vb.getVaultPath() == SecureVault("wallet-cold.dat").getVaultPath());
+    vb.setWalletScope("wallet.dat");
+    EXPECT_TRUE(endsWith(vb.getVaultPath(), "vault.dat"));
+}
+
 void testWalletSecurityController()
 {
     using dragonx::services::WalletSecurityController;
@@ -6390,6 +6419,7 @@ int main()
     testNetworkRefreshRpcCollectors();
     testNetworkRefreshResultModels();
     testOperationStatusPollParsing();
+    testSecureVaultScope();
     testWalletSecurityController();
     testWalletSecurityWorkflow();
     testWalletSecurityWorkflowExecutor();