Compute v5 transaction IDs locally per ZIP 244

[zmanian]

Summary

• Implements ZIP 244 BLAKE2b-256 hash tree for computing non-malleable v5 transaction IDs during parsing, resolving the core of issue Implement ZIP 244 txid logic for v5 transactions #392 (filed May 2022)
• Vendors the pure-Go BLAKE2b implementation from golang.org/x/crypto@v0.45.0 into parser/internal/blake2b/, adding personalization parameter support (~3 lines of modification to the digest struct and Reset())
• Re-parses Transaction.rawBytes in computeV5TxID() to feed fields into 18 personalized BLAKE2b-256 digests matching the ZIP 244 hash tree, including correct handling of the v5 shared sapling anchor (buffered and replayed per-spend in the noncompact digest)
• All 10 test vectors from testdata/tx_v5.json now validate txids (previously skipped with a TODO comment)

Motivation

lightwalletd returns incorrect txids for v5 transactions. The workaround (PR #393) fetches txids from zcashd/zebrad via a verbose getblock RPC call instead of computing them locally. This creates a trust dependency on the backing node for txid correctness. The core blocker was that Go's golang.org/x/crypto/blake2b package doesn't support the personalization parameter required by ZIP 244.

Changes

File	Action	Description
parser/internal/blake2b/blake2b.go	New	Vendored BLAKE2b-256 with personalization support
parser/internal/blake2b/compress.go	New	Vendored pure-Go compression function (unmodified)
parser/internal/blake2b/LICENSE	New	BSD license from golang.org/x/crypto
parser/internal/blake2b/blake2b_test.go	New	Standard + personalized test vectors (verified with Python hashlib)
parser/zip244.go	New	ZIP 244 hash tree: computeV5TxID + sub-functions for header, transparent, sapling, orchard digests
parser/zip244_test.go	New	Tests against all 10 tx_v5.json vectors, both standalone and via ParseFromSlice
parser/transaction.go	Modified	7 lines added to ParseFromSlice to call computeV5TxID for v5 transactions
parser/transaction_test.go	Modified	Replaced "can't check txid" comment with actual txid validation

Deferred to follow-up PRs

• Remove the verbose RPC workaround in common.go:getBlockFromRPC (simplify to single getblock verbose=0 call)
• Pre-v5 txid computation (double-SHA256 of rawBytes for v1-v4)

Test plan

• go test ./parser/internal/blake2b/... -- blake2b unit tests pass (standard + personalized vectors)
• go test ./parser/... -- all parser tests pass including new txid validation against 10 test vectors
• go vet ./parser/... ./parser/internal/blake2b/ -- clean
• go build ./... -- clean build
• Manual verification: compare computed txids against a block explorer for a sample of real mainnet v5 transactions

Closes #392

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