wallet: resolve the autoshield destination at startup

pwalletMain->autoShieldAddress was only ever written by a round running in the
current process, so on any node without an explicit -autoshieldaddress,
z_autoshieldstatus reported an empty destination from startup until the first
round fired. disabled_reason was empty on that path too (rpcwallet.cpp), so the
RPC showed autoshield true, running false, no address and no explanation -- the
exact silent state z_autoshieldstatus was added to eliminate. On a restored
wallet, which pre-derives the whole -mnemonicsaplinggap window and therefore
always has an in-gap account to pick, the answer was known at startup and simply
not computed.

Split the read-only half of resolveDestination into a free function shared with
init: the configured override if set, else the lowest in-gap account
m/32'/coin'/i' the wallet already holds. init calls it once when autoshield is
enabled and no explicit address was given.

It deliberately does not generate a key. Deriving a fresh sapling account as a
side effect of populating a status field would mutate the wallet to make an RPC
prettier, so step 3 of resolveDestination -- the generation path, which must stay
inside the operation where an unlocked wallet is already established -- is left
where it was. A brand-new wallet holds nothing in-gap, so the field stays empty
there and disabled_reason now says why instead of being blank.

Behaviour is otherwise unchanged: same derivation, same lowest-index-wins rule,
same refusal to trust CKeyMetadata, same caching for the life of the process.

Verified on an isolated regtest chain, 12/12:
  fresh wallet      -> address "", reason "no destination resolved yet; one will
                       be derived from the HD seed on the first round"
  after one round   -> address set, reason empty
  after RESTART     -> address visible with NO block mined since (height 12 both
                       sides), and z_listaddresses still holds exactly 1 address,
                       so init derived nothing
  -autoshieldaddress-> still overrides the derived destination, and the round
                       shields into it

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/wallet/rpcwallet.cpp

@@ -3415,7 +3415,7 @@ UniValue z_autoshieldstatus(const UniValue& params, bool fHelp, const CPubKey& m
         why = strprintf("deferred while %s is running; rounds resume when it finishes",
                         pwalletMain->fSweepRunning ? "z_sweep" : "sapling consolidation");
     } else if (pwalletMain->autoShieldAddress.empty()) {
-        why = "";
+        why = "no destination resolved yet; one will be derived from the HD seed on the first round";
     }
     ret.push_back(Pair("disabled_reason", why));