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/data/wallet_state.h

@@ -318,6 +318,12 @@ struct WalletState {
     
     // Timestamps for refresh logic
     int64_t last_balance_update = 0;
+    // A Core poll that skipped the balance confirmed it is still current (nothing it depends on changed):
+    // the staleness badge counts from the later of the two.
+    int64_t balance_verified_at = 0;
+    // The last balance call that was sent failed (or timed out): the staleness badge then shows at its usual
+    // thresholds even while the scan budget is holding the next one.
+    bool balance_last_attempt_failed = false;
     int64_t last_address_update = 0;   // set when an address-list refresh applies; 0 = never loaded yet
     int64_t last_tx_update = 0;
     int64_t last_peer_update = 0;
@@ -362,6 +368,8 @@ struct WalletState {
         // badge (and any "updated X ago" reader) briefly reports it as current until the first refresh
         // re-stamps it. All readers treat 0 as "never" (formatTimeAgoShort/timeAgo return "").
         last_balance_update = last_address_update = last_tx_update = last_peer_update = last_mining_update = 0;
+        balance_verified_at = 0;
+        balance_last_attempt_failed = false;
     }
     
     // Rebuild combined addresses list from z/t lists