mod: update to btcd v2 modules

[ellemouton]

Summary

Migrates lndclient to the new per-package v2 modules introduced by
btcd v0.26.0. The monolithic btcd packages (wire, txscript,
chaincfg, chainhash, btcutil, psbt) were split out into their own
/v2 modules, and the address-handling symbols moved out of btcutil
into a new address package.

Changes

• Bump btcd to v0.26.0 and adopt the v2 modules: wire/v2,
  txscript/v2, chaincfg/v2, chainhash/v2, btcutil/v2, psbt/v2,
  and btcec/v2 v2.5.0.
• The address symbols that moved from btcutil into the new address
  package (DecodeAddress, the Address types, the NewAddress*
  constructors) are imported as btcaddr to avoid shadowing by local
  address variables.
• btcec/v2 v2.5.0 requires Go 1.25, so the go directive is bumped to
  1.25.11.

Dependencies

Until the migrated modules are tagged, the dependency chain is pulled in
via temporary replace directives:

• btcd v0.26.0 (tagged)
• lightningnetwork/lnd (btcd v2 migration)
• btcsuite/btcwallet (btcd v2 migration)
• lightninglabs/neutrino (btcd v2 migration)
• lightningnetwork/lightning-onion (btcd v2 migration)

Once those are tagged the replace directives will be dropped and the
go.mod pinned to the released versions.

Warning

Draft — do not merge until the upstream dependencies above are tagged.

[ellemouton]

Combined into #280 for easier review — that PR now carries both the btcd v2 module migration and the WalletKit.SubmitPackage client method as two commits.