[v1] Live RPC conformance: freeze v1 method subset

[SMC17]

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

Scope

• Freeze the v1 live RPC claim to a small supported subset.
• Ensure every declared method has the needed positive and negative contracts.

Files

• docs/RPC_CONTRACT_INDEX.md
• test_data/rpc_live_methods_schema.json
• src/parity_check.zig

Gates

• Gates C/D

Acceptance Criteria

• Live subset is exactly server_info, fee, ledger, ledger_current, account_info, submit.
• Every method has positive coverage and required negative contracts.
• No extra method is marketed as release-ready.

Related

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