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

[czarcas7ic]

Motivation

ZIP-244 transaction IDs and authorizing-data digests recompute fixed BLAKE2b hashes whenever transparent, Sapling, Orchard, or Ironwood bundle nodes are absent.

Solution

Use verified constants for those empty digest nodes and for missing Sapling spend or output children. A unit test recomputes every constant from its personalization.

The digest benchmark now covers both an Orchard-only V5 transaction and an Ironwood-only V6 transaction. The V6 case leaves the transparent, Sapling, and Orchard bundles empty.

Benchmark

Using the same Ironwood-only V6 benchmark harness on main at 819ef11 and this branch:

• auth_digest: 5.2405 us to 5.0703 us, 2.8% faster (95% CI: 2.5% to 3.1%)
• txid_and_auth_digest: 7.3915 us to 6.9712 us, 5.6% faster (95% CI: 5.5% to 5.8%)

Rerun with:

cargo bench -p zakura-chain --bench transaction --locked -- ironwood_only

Testing

• cargo fmt --all -- --check
• cargo test -p zakura-chain zip244 --locked
• cargo bench -p zakura-chain --bench transaction --locked -- ironwood_only
• cargo clippy -p zakura-chain --all-targets --locked -- -D warnings

[czarcas7ic]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep it up!

Reviewed commit: 56bda41b0b

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[github-actions]

Zakura PR node run — tip/mainnet ✅

Verdict	ok
Commit	bae4e0304c2e69ddd89becc1bab7dc17471c6045
Duration	60 min
Height	3417514 → 3419382 (+1868)
Blocks/hour	1868
Estimated tip at end	3419382
Peers (min/last)	8 / 17
Peak RSS	628.7 MiB
Service at end	active, 0 restart(s)
Log errors/warns/panics	0 / 3 / 0

Height over time

Elapsed	Height	Peers	RSS (MiB)
0m	3417514	9	277.6
5m	3419254	9	616.1
9m	3419339	9	623.9
14m	3419342	10	624.9
18m	3419351	10	625.1
23m	3419356	10	625.5
27m	3419358	13	625.9
32m	3419361	13	626.3
36m	3419364	13	626.8
41m	3419369	14	626.9
45m	3419372	14	627.1
50m	3419373	15	627.4
54m	3419379	15	627.9
59m	3419381	17	628.7
59m	3419382	17	628.7

Notes

• State DB format v28 matches the PR tree.
• Incremental build took 259s (warm baked cache).

Droplet deleted after the run.

Workflow run