perf(transaction): cache empty ZIP-244 bundle hashes

[ValarDragon]

Motivation

ZIP-244 txid and authorizing-data digests use bare BLAKE2b personalization hashes when a bundle is absent. The native Zebra implementation recomputed those empty hashes on each v5/v6 transaction even though the values are fixed by the personalization strings.

Solution

• Add private byte constants for the empty transparent, Sapling, Orchard, and Ironwood ZIP-244 txid/auth digest nodes.
• Feed those constants directly into the parent txid/auth hash when a bundle is absent.
• Keep a unit test that recomputes every constant from its personalization string.
• Add an Orchard-only ZIP-244 digest benchmark with 1000 samples.

Benchmarks

Command:

cargo bench -p zebra-chain --bench transaction auth_digest/orchard_only

Exact origin/ironwood-main baseline with the benchmark added:

Benchmark	Baseline	This PR	Speedup
auth_digest/orchard_only	8.3584 us	8.0284 us	~4.0%
txid_and_auth_digest/orchard_only	11.892 us	11.194 us	~5.9%

Both runs used 1000 Criterion samples.

Validation

• cargo fmt --all
• cargo test -p zebra-chain zip244
• cargo bench -p zebra-chain --bench transaction --no-run
• cargo bench -p zebra-chain --bench transaction auth_digest/orchard_only

AI Disclosure

Used Codex to implement the constants, benchmark, tests, and PR description.

[v12-auditor]

Note

Complete: Audit complete. V12 did not find any issues that need review.

Open the full results here.

Analyzed two files, diff e3571d1...c8741a8.