[v1] Canonical field ordering for supported transaction set

[SMC17]

Problem
The v1 release needs this work to make the toolkit claim defensible.

Scope

• Replace simplified transaction serialization with canonical XRPL field ordering for the supported v1 transaction set.
• Supported set: Payment, AccountSet, OfferCreate, OfferCancel.

Files

• src/transaction.zig
• src/canonical_tx.zig
• src/serialization.zig

Gates

• Gate B

Acceptance Criteria

• Supported transactions serialize in canonical XRPL field order.
• No simplified serializer remains on the release path.
• Deterministic vectors are added for every supported type.

Related

• Epic: [Epic] v1 XRPL Toolkit release #45
• Milestone: v1 XRPL Toolkit

[SMC17]

Started in working tree:

• audited the current release-path serialization flow
• replaced PaymentTransaction.serialize in src/transaction.zig from raw byte concatenation to the canonical serializer path
• current implementation now emits canonical ordering for tx type, account, sequence, fee, destination, amount, and optional destination tag

Remaining work is to extend the same treatment across the full declared v1 supported transaction set and tighten vector coverage in Gate B.

[SMC17]

Progress update:

• extended canonical serialization beyond PaymentTransaction to AccountSetTransaction, OfferCreateTransaction, and OfferCancelTransaction in src/transaction.zig
• added canonical serialization tests for all four v1 transaction structs
• kept unsupported IOU and unsupported AccountSet field encodings explicit rather than silently faking them

Verification:

• scripts/gates/gate_a.sh artifacts/gate-a-local-20260315c passes locally
• scripts/gates/gate_b.sh artifacts/gate-b-local-20260315b passes locally