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>

tools/fake_dragonxd/README.md

@@ -122,6 +122,7 @@ inherits the GUI's environment.
 | `FAKE_STUCK_SHUTDOWN` | 0 | the Sep 29 orphan: hold the datadir lock, no listener, ignore SIGTERM, never exit |
 | `FAKE_WALLET_NOTES` | (off) | a note wallet: `addr:count:amount,...` shielded notes; enables `z_mergetoaddress` + its operations (dragonxd rules: confirmed unlocked notes, a batch locked on return, merged note next block) |
 | `FAKE_MERGE_SECS` | 6 | how long a `z_mergetoaddress` operation runs before it succeeds |
+| `FAKE_METHODS_ALL` | 0 | `1` = one `call m=<method> p=<params>` event per request (the GUI's RPC cadence; the `methods` lines keep only the top 10 counts) |
 | `FAKE_HEIGHT`, `FAKE_REBUILD_TOTAL`, `FAKE_NOTES`, `FAKE_VERSION` | 3327145, 62583, 13632, 1030050 | cosmetic numbers |
 
 ## Test cases

tools/fake_dragonxd/fake_dragonxd.cpp

@@ -749,6 +749,13 @@ json execOne(const json& req, int& status)
     json params = req.contains("params") ? req["params"] : json::array();
     if (!params.is_array()) params = json::array();
     status = 200;
+    // FAKE_METHODS_ALL=1: one "call" event per request (method + params), to see the GUI's RPC cadence.
+    static const bool logAll = [] { const char* v = std::getenv("FAKE_METHODS_ALL"); return v && std::string(v) == "1"; }();
+    if (logAll) {
+        std::string p = params.dump();
+        if (p.size() > 160) p = p.substr(0, 160) + "...";
+        event("call", "m=" + m + " p=" + p);
+    }
 
     // Like dragonxd: stop and help take no lock, nor does getrescaninfo (it only reads wallet fields).
     const bool lockFree = (m == "stop" || m == "help" || m == "getrescaninfo");