[v1] Narrow and harden submit to the supported transaction set

[SMC17]

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

Scope

• Narrow submit handling to the supported v1 transaction set.
• Make unsupported transaction shapes fail clearly and deterministically.

Files

• src/rpc.zig
• src/rpc_methods.zig
• test_data/rpc_live_negative_schema.json

Gates

• Gates C/D

Acceptance Criteria

• Only supported transaction types are accepted on the release path.
• Unsupported shapes fail with deterministic errors.
• Docs and schema contracts match actual behavior.

Related

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