1/5: Enrich DecryptedOutput with nullifier and position metadata

[czarcas7ic]

Summary

• Add TxBundlePositions, compute_enriched_outputs(), and collect_wallet_note_positions() to the decrypt module for enriching decrypted outputs with nullifier bytes and commitment tree positions post-decryption
• Add WalletWrite::notify_wallet_note_positions and prune_tracked_nullifiers trait methods with default no-op implementations for backends to override
• Add ScanningKeys::from_account_ufvks_with_scopes for selective scope inclusion during batch scanning
• Change ReceivedShieldedOutput::nullifier() to return an owned value instead of a borrowed one
• Tighten the enrichment API so compute_enriched_outputs() derives the transaction reference from DecryptedTransaction, and re-export the new helpers from the crate root so this slice is independently lint-clean
• Wire notify_wallet_note_positions through the SQLite and memory backends now that the high-level WalletWrite hook exists, and add the owning backend changelog entry in this branch

Context

This is PR 1/5 in a stacked PR series implementing External-IVK-only batch scanning. This PR adds the API foundations that the rest of the stack builds on, plus the concrete high-level hook overrides needed to keep the new WalletWrite contract from being a silent no-op on backends that already have the underlying scan-complete machinery.

Full stack:

1. This PR — API foundations
2. adam/sync-enhancement-pipeline — Core behavioral change
3. adam/sqlite-enhancement-support — SQLite backend implementation
4. adam/memory-backend-enhancement — Memory backend implementation
5. adam/external-ivk-scan-tests — Tests and changelogs

Review focus

• compute_enriched_outputs() API shape in decrypt.rs and whether deriving the transaction from DecryptedTransaction is the right invariant
• The nullifier() return type change and its downstream compatibility
• The new WalletWrite hook contract and the concrete SQLite / memory overrides for notify_wallet_note_positions

Test plan

• cargo clippy -p zcash_client_backend --all-features -- -D warnings
• cargo check -p zcash_client_sqlite --all-features
• cargo check -p zcash_client_memory --all-features