wallet: add default-on auto-shield-coinbase; drain miner coinbase into a z-addr

On this ac_private=1 chain miners accumulate one transparent coinbase UTXO per
block (the only transparent output the chain permits). It had to be shielded
manually via z_shieldcoinbase, and left unshielded it is the sole persistent
metadata leak on the chain and the source of miner UTXO-fragmentation send
failures.

Add AsyncRPCOperation_autoshieldcoinbase: a periodic, default-on wallet op
driven from CWallet::ChainTip alongside sweep/consolidation, draining matured
coinbase into a wallet-owned Sapling z-address in size-bounded batches.

- Enqueue-only driver (RunAutoShieldCoinbase): takes only cs_wallet in the
  notify context; all gathering runs on the async worker under LOCK2(cs_main,
  cs_wallet).
- Dedicated op (not a reuse of z_shieldcoinbase) so it never toggles mining.
- Default-ON but conditional: silent no-op on -disablewallet, external
  -mineraddress, non-mining, or locked wallets (explicit IsLocked() guard).
- Destination is reuse-then-create; -autoshieldaddress override is
  spend-key-validated at init so funds cannot be stranded.
- Sweep-model bookkeeping: advances nextAutoShield and clears the running flag
  on every terminal state; an op-id guard stops a stale op clobbering scheduler
  state; a self-guard stops cancel/re-enqueue churn.
- Sietch-padded output shape matches manual z_shieldcoinbase txns.

Config: -autoshield (default true), -autoshieldinterval, -autoshieldaddress,
-autoshieldfee (range-validated), -autoshieldminutxos.

Incorporates fixes from an 8-angle code review: CAmount fee type with init-time
range validation, op-id-guarded flag bookkeeping, driver self-guard, and a
tipHeight-consistent NU-activation guard.

Note: the fConsolidationRunning / nextConsolidation consolidation-scheduler
wedge is a pre-existing bug, left for a separate change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/Makefile.am

@@ -243,6 +243,7 @@ BITCOIN_CORE_H = \
   wallet/asyncrpcoperation_mergetoaddress.h \
   wallet/asyncrpcoperation_saplingconsolidation.h \
   wallet/asyncrpcoperation_sweep.h \
+  wallet/asyncrpcoperation_autoshieldcoinbase.h \
   wallet/asyncrpcoperation_sendmany.h \
   wallet/asyncrpcoperation_shieldcoinbase.h \
   wallet/crypter.h \
@@ -321,6 +322,7 @@ libbitcoin_wallet_a_SOURCES = \
   wallet/asyncrpcoperation_mergetoaddress.cpp \
   wallet/asyncrpcoperation_saplingconsolidation.cpp \
   wallet/asyncrpcoperation_sweep.cpp \
+  wallet/asyncrpcoperation_autoshieldcoinbase.cpp \
   wallet/asyncrpcoperation_sendmany.cpp \
   wallet/asyncrpcoperation_shieldcoinbase.cpp \
   wallet/crypter.cpp \