Phase 1: descriptor-backed standard wallet address derivation (wpkh)

[Copilot]

Migrates standard wallet receive/change address derivation to use BIP-84 wpkh descriptors internally, while keeping all existing interfaces, signing paths, and Angor custom scripts unchanged. Old wallets auto-migrate on first scan without any user action.

Model changes (AccountInfo)

Three new nullable fields alongside existing ExtPubKey/Path (additive – no breaking storage change):

• ReceiveDescriptor – wpkh([fp/84h/coinTypeh/0h]xpub/0/*)
• ChangeDescriptor – wpkh([fp/84h/coinTypeh/0h]xpub/1/*)
• MasterFingerprint – first 4 bytes of HASH160(root pubkey), lowercase hex

New helper: StandardWalletDescriptors

Static class in Angor.Shared:

• Build(xpub, fingerprint, coinType) → (ReceiveDescriptor, ChangeDescriptor)
• ExtractXPub(descriptor) → account-level xpub string
• IsChangeBranch(descriptor) → bool
• TryMigrate(accountInfo, coinType) → generates descriptors from legacy ExtPubKey when absent

WalletOperations changes

• BuildAccountInfoForWalletWords now computes the master fingerprint and populates all three descriptor fields on creation.
• UpdateAccountInfoWithNewAddressesAsync calls TryMigrate at entry (no-op if already migrated), then sources the scanning xpub from the descriptor rather than raw ExtPubKey:

// Before
var (index, items) = await FetchAddressesDataForPubKeyAsync(
    accountInfo.LastFetchIndex, accountInfo.ExtPubKey, network, false);

// After – xpub sourced from descriptor; falls back to ExtPubKey for unmigrated records
var receiveXPub = !string.IsNullOrEmpty(accountInfo.ReceiveDescriptor)
    ? StandardWalletDescriptors.ExtractXPub(accountInfo.ReceiveDescriptor)
    : accountInfo.ExtPubKey;
var (index, items) = await FetchAddressesDataForPubKeyAsync(
    accountInfo.LastFetchIndex, receiveXPub, network, false);

Migration behaviour

Descriptor derivation uses the same account xpub at m/84'/coinType'/0' as the legacy path, so all addresses are identical. Old wallets transparently gain descriptors in-memory on first UpdateAccountInfoWithNewAddressesAsync call; no manual migration or data loss.

Tests (WalletDescriptorTests – 23 cases)

• Descriptor format correctness with/without fingerprint
• ExtractXPub round-trip
• BuildAccountInfoForWalletWords integration
• Descriptor-derived addresses match legacy derivation for each index and branch
• TryMigrate lifecycle: generates, is idempotent, skips when xpub absent

Not changed

Angor custom scripts, founder/investor derivation, Nostr paths, signing, all public service interfaces.

Recommended follow-up

• Persist migrated descriptors to storage (currently in-memory per session)
• Phase 2: Taproot (tr(…)) descriptor support

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