perf(litebe): change-gate the idle lite refresh worker

The background 2 s refresh worker re-queried and re-copied the whole
wallet every cycle (a fresh TLS refresh plus a multi-megabyte JSON
build, copies and DOM parse), even when nothing had changed, so CPU and
the network never went idle.

The worker now skips the full refresh and the re-publish on a cycle that
is provably unchanged, while never letting the UI show stale confirmed
balance or transactions:

- The lite bridge keeps a monotonic mutation epoch, bumped after any
  command not on a read-only allow-list, so a local change made on any
  thread (send, shield, import, new address, encrypt/unlock/lock,
  console) is detected at one choke point.
- Each cycle the worker runs one cheap sync probe (one connection, the
  mandatory block ingest) and a local height read, and refreshes only
  when the scanned height advanced, the mutation epoch moved, the
  encryption state changed, a sync is in progress, or a ~10 s heartbeat
  elapsed; otherwise it skips. Direct/synchronous callers are not gated.
- A failed probe backs off instead of running a second sync, so a dead
  server is not retried back-to-back.

Confirmed funds can never be stale beyond one cycle (every confirmed
change advances the height or bumps the epoch); only unconfirmed mempool
rows can lag up to the heartbeat, and the spend path re-queries the
backend directly rather than reading the gated model. Average idle cost
drops from three data connections per cycle to about one.

Tests: a bridge mutation-epoch test and a worker change-gate test
(skip-when-unchanged; refresh on height advance, local mutation and
encryption change) against the fake backend.

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

tests/fake_lite_backend.h

@@ -43,6 +43,7 @@ inline std::atomic<long> g_liteFakeSyncCalls{0};
 inline std::atomic<bool> g_liteFakeBadBalance{false}; // when true, "balance" returns invalid JSON
 inline std::atomic<bool> g_liteFakeSendFails{false};  // when true, "send"/"shield" return {"error":..}
 inline std::atomic<long> g_liteFakeSaveCount{0};      // counts "save" commands (persistence checks)
+inline std::atomic<unsigned long long> g_liteFakeHeight{1000};  // settable last-scanned height (C23 gate test)
 inline std::atomic<bool> g_liteFakeEncrypted{false};  // wallet-encryption state machine (encrypt/...)
 inline std::atomic<bool> g_liteFakeLocked{false};     // spending-keys-locked state
 inline std::atomic<bool> g_liteFakeBlockNew{false};      // "new" blocks (models do_save waiting on sync_lock)
@@ -87,6 +88,7 @@ inline void resetLiteFakeCounters()
 {
     g_liteFakeAlloc = 0;
     g_liteFakeFreed = 0;
+    g_liteFakeHeight = 1000;
     g_liteFakeShutdownCalled = false;
     g_liteFakeDeadServerSubstr.clear();
     g_liteFakeWarmupServerSubstr.clear();
@@ -215,7 +217,8 @@ inline char* liteFakeExecute(const char* command, const char* args)
         if (std::strcmp(c, "list") == 0)
             return liteFakeDup("[{\"txid\":\"faketx\",\"datetime\":1700000000,\"block_height\":990,"
                                "\"unconfirmed\":false,\"address\":\"zs1fakeaddr\",\"amount\":150000000,\"memo\":\"\"}]");
-        if (std::strcmp(c, "height") == 0) return liteFakeDup("{\"height\":1000}");
+        if (std::strcmp(c, "height") == 0)
+            return liteFakeDup(("{\"height\":" + std::to_string(g_liteFakeHeight.load()) + "}").c_str());
         if (std::strcmp(c, "info") == 0)
             return liteFakeDup("{\"chain_name\":\"main\",\"version\":\"sdxl-fake\",\"latest_block_height\":1000}");
         // M4 spend/backup commands. send/shield report failure via {"error":..} (NOT an