2/5: External-only scanning with sync loop enhancement pipeline

[czarcas7ic]

Summary

• Switch scan_cached_blocks to use only Scope::External IVKs, halving key-agreement work per output
• Restructure the sync loop: run() / running() now service enhancement requests between scan chunks via ServiceOutcome (Drained / Stabilized)
• store_raw_transaction enriches decrypted outputs with nullifiers and tree positions, then calls notify_wallet_note_positions
• Nullifier pruning is deferred from put_blocks to the sync orchestrator so enhancement can still consult the tracked-nullifier map
• Carry the minimal SQLite queue semantics this slice depends on: mined transactions with raw are queued as Enhancement (not GetStatus), and transaction_data_requests() is returned in deterministic order for the stabilized-queue detection
• Restore an explicit safe path for non-sync::run callers by having scan_cached_blocks() prune automatically only when no follow-up transaction-data requests were enqueued, and add focused regressions for both that behavior and the transparent mempool cursor-advance edge case

Context

This is PR 2/5 — the core behavioral change. It depends on PR 1 for the enrichment API, includes the minimum SQLite queue behavior needed for the new sync logic to be correct in isolation, and now also makes the direct-scan_cached_blocks pruning contract explicit for non-sync callers.

Full stack:

1. adam/enriched-decrypted-output — API foundations
2. This PR — 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

• Drained / Stabilized termination logic in sync.rs
• store_raw_transaction enrichment and note-position notification flow
• The deterministic SQLite queue behavior that makes stabilized-request detection and late change recovery valid in this slice
• The direct-scan pruning contract and the transparent mempool request edge case

Test plan

• cargo check -p zcash_client_backend --features sync
• cargo test -p zcash_client_backend --all-features scan_cached_blocks_prunes_when_request_queue_is_empty
• cargo test -p zcash_client_backend --all-features scan_cached_blocks_defers_pruning_when_requests_are_pending
• cargo test -p zcash_client_backend --all-features mempool_all_request_does_not_advance_address_checked
• cargo test -p zcash_client_backend --all-features mined_all_request_advances_address_checked