wallet/init: decrypt every wallet note into the plaintext cache at startup

The plaintext cache is in memory only, so a restart empties it. Without
a warm-up, the first balance call after a restart decrypts the whole
wallet under LOCK2(cs_main, cs_wallet): ~6s for a 12k-note wallet. That
is the observed failure: restart, the GUI connects, and the node looks
stuck.

CWallet::WarmSaplingNoteCache decrypts every note whose plaintext isn't
cached, spent ones included, because z_listreceivedbyaddress
(ignoreSpent=false) is called once per address by the GUI's history
scan.

init runs it at the end of the wallet block, just before
SetBroadcastTransactions. At that point RPC is still in warmup (it
shows "Decrypting wallet notes...") and the wallet notification
thread, whose startup witness rebuild can hold cs_wallet for a long
time, has not started yet. It takes cs_wallet only. -notecachewarmup=0
(debug help) skips it, which leaves the lazy fill in GetFilteredNotes
to do the work.

Unlike GetFilteredNotes, the warm-up never asserts. A note that doesn't
decrypt under its ivk, or names an output its tx lacks, is counted in
the summary line and listed under -debug=zrpcunsafe. A corrupt note
therefore can't stop the node from starting, or from serving the
key-export RPCs allowed during warmup; GetFilteredNotes still asserts
on it, as before.

Tests:
- WarmUpFillsEveryNote: spent, locked, unconfirmed and stale-block
  notes included; walks afterwards decrypt nothing.
- WarmUpSkipsBadNotes: undecryptable and out-of-range notes are skipped
  without aborting.

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

src/wallet/wallet.h

@@ -1497,6 +1497,9 @@ public:
     /** Notes FillSaplingNoteCache has decrypted since startup (FindMySaplingNotes' fills not counted).
      *  Guarded by cs_wallet. */
     uint64_t nSaplingNoteCacheFills = 0;
+    /** Decrypts every wallet note whose plaintext isn't cached yet (all of them after wallet.dat is loaded).
+     *  Never asserts: a note that won't decrypt is logged and left unfilled. Run at startup. */
+    void WarmSaplingNoteCache();
 
     void GetFilteredNotes(std::vector<SaplingNoteEntry>& saplingEntries,
                           std::string address,