perf(net): one scan budget for all wallet scans, gated on what changed

The balance, address scan, history and probe were throttled one by one at
10% each, which added up to 30-40% of dragonxd's cs_main with all of them
due. services::WalletScanGovernor runs one wallet scan at a time across
both RPC lanes and keeps one shared quiet clock at 15% of wall-clock (cut
when the node is behind). Waiters are served first in, first out, with a
head start for what the page shows and a starvation floor of max(30 s
visible / 900 s hidden, 15x the stream's cost). Explicit work (a dirty
set, the user's send, a first load) is never refused but is charged.

Routine scans run only when the tip, the mempool size (getmempoolinfo:
no cs_main) or a wallet-mutation epoch changed since their last good scan,
or a safety sweep is due. Pages that don't show the balance poll it at
most every max(10 min, 50x its cost) (2 min with client-side auto-shield);
coming back into view with an old value fetches it at once. The staleness
badge counts from the last poll that confirmed nothing changed, and hides
while the budget holds the balance within its guarantee.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/rpc/rpc_busy_gate.h

@@ -47,7 +47,8 @@ public:
     // Methods dragonxd runs without cs_main: always sent, and their results say nothing about cs_main.
     static bool isLockFree(const std::string& method)
     {
-        return method == "stop" || method == "help" || method == "getrescaninfo";
+        // getmempoolinfo takes only mempool.cs (rpc/blockchain.cpp), and the dispatcher no global lock.
+        return method == "stop" || method == "help" || method == "getrescaninfo" || method == "getmempoolinfo";
     }
 
     // May this request be sent now? Lock-free methods always; the rest only while the node isn't busy.