consensus: pass spent KIs to cuprated

[redsh4de]

What

Addresses the FIXME in add_valid_block_to_main_chain.

Why

Block verification in consensus walks every transaction's inputs to dedupe and check key images. After verification, cuprated walked them a second time to notify the txpool.

This makes it so that the KI vec is returned from consensus, eliminating the second walk.

Where

consensus, helper, cuprated

How

In helper:

• New tx_key_images() helper for extracting a transactions key images

In consensus:

• Batch preparation now returns TxsWithKis per block (transactions + validated key images).
• The verify functions now return VerifiedBlock (wrapper for verified block info + the spent key images).
• verify_prepped_main_chain_block takes a PreparedBlockTxs::{Checked, Unchecked} enum so the caller can say whether key images still need checking:
  • Unchecked(Vec<TransactionVerificationData>) - single-block path. Extracts KIs and checks.
  • Checked(TxsWithKis) - batch-sync path. TxsWithKis is only produced by batch_prepare_main_chain_blocks where validation was already done, so we skip checking here and use the passed KIs as is.
• Split FullVerification into two methods, so the txpool path doesn't allocate a key-image Vec it would discard:
  • verify (txpool): returns just the transactions.
  • verify_with_kis (block, internal): returns transactions and key images.
• Key-image checking seperated into three:
  • check_kis: used by the batch & txpool paths. Takes pre-computed key images.
  • extract_and_check_kis: single-block path. Walks inputs once and builds both the KI dedup set and Vec in one pass.
  • check_kis_unspent: shared chain-spent db query, skipped on a empty set.
• key_images_spent_checked bool removed from BatchPrepareCache. The existence of TxsWithKis means that the key images were checked, so this becomes redundant.
• Capacity hint change: check_kis_unique's old with_capacity(size_hint().1.unwrap_or(0) * 2) always resolved to zero for the batch caller, because when flat_map is used on a slice iter, the upper-bound size hint is always None. To avoid rehashing, we count the KIs once up front and pass capacity as a param.
• New alt_block_to_verified_block replaces cuprated's alt_block_to_verified_block_information. Computes total_fees and spent_key_images in one pass, then constructs a VerifiedBlock.

In cuprated:

• add_valid_block_to_main_chain splits the new VerifiedBlock into block information and key images, used like before.
• The reorg-reverse path now calls the new alt_block_to_verified_block in consensus; the local helper is removed.

[redsh4de]

Diagram of how key images flow and where checks happen on each path in this PR.
It can be seen that with this, we have a guarantee that we will only extract key images once at the earliest possible step for each path, and get them passed down to the block handler