feat(cold): add get_receipt_with_context composite query

[prestwich]

Summary

• Adds ReceiptContext struct bundling header, transaction, receipt, confirmation metadata, and prior cumulative gas
• Adds get_receipt_with_context to the ColdStorage trait for single-roundtrip RPC receipt building
• Implements for both MemColdBackend and MdbxColdBackend
• Adds conformance test covering block+index lookup, tx hash lookup, prior gas computation, and missing-block case
• Removes BlockTag enum and HeaderSpecifier::Tag variant — dead code since the only "latest block" consumer uses get_latest_block(), and Finalized/Safe were never written
• Trims MetadataKey to only LatestBlock, removing FinalizedBlock, SafeBlock, and EarliestBlock

Test plan

• cargo clippy -p signet-cold --all-features --all-targets
• cargo clippy -p signet-cold --no-default-features --all-targets
• cargo clippy -p signet-cold-mdbx --all-features --all-targets
• cargo clippy -p signet-cold-mdbx --no-default-features --all-targets
• cargo +nightly fmt
• cargo t -p signet-cold — conformance passes for MemColdBackend
• cargo t -p signet-cold-mdbx --all-features — conformance passes for MdbxColdBackend
• cargo t -p signet-storage — UnifiedStorage still compiles and passes

🤖 Generated with Claude Code

[prestwich]

[Claude Code]

Integrating ReceiptContext

What it provides

ReceiptContext bundles everything an RPC layer needs to build a complete eth_getTransactionReceipt response from a single cold storage call:

Field	Type	Purpose
header	Header	Block header (for block_hash, block_number, etc.)
transaction	TransactionSigned	The originating transaction (for from, to, type, etc.)
receipt	Confirmed<Receipt>	Receipt wrapped with ConfirmationMeta (block number, block hash, tx index)
prior_cumulative_gas	u64	Cumulative gas of all preceding txs — subtract from the receipt's cumulative_gas_used to get per-tx gas_used

Querying

Use either handle type (ColdStorageHandle or ColdStorageReadHandle):

// By transaction hash
let ctx = handle
    .get_receipt_with_context(ReceiptSpecifier::TxHash(tx_hash))
    .await?;

// By block number + tx index
let ctx = handle
    .get_receipt_with_context(ReceiptSpecifier::BlockAndIndex { block: 42, index: 0 })
    .await?;

Both return ColdResult<Option<ReceiptContext>> — None when the receipt doesn't exist.

Computing per-transaction gas

let gas_used = ctx.receipt.inner().inner.cumulative_gas_used - ctx.prior_cumulative_gas;

prior_cumulative_gas is 0 for the first transaction in a block, so the subtraction always works.

Accessing confirmation metadata

ConfirmationMeta is carried inside Confirmed<Receipt>:

let meta = ctx.receipt.meta();
let block_number = meta.block_number();
let block_hash  = meta.block_hash();
let tx_index    = meta.transaction_index();

Backend performance

The MDBX backend (signet-cold-mdbx) overrides the default implementation to serve get_receipt_with_context in a single read transaction, avoiding the multiple roundtrips that the default trait implementation requires. The in-memory backend uses the default composed implementation.

Trait implementors

If you're writing a custom ColdStorage backend, get_receipt_with_context has a default implementation that composes get_receipt, get_header, and get_transaction. Override it only if your backend can serve the query more efficiently (e.g., in a single database transaction).