[v1] Add toolkit CLI commands

[SMC17]

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

Scope

• Add a toolkit CLI so the release artifact is usable outside tests.
• Commands should exercise the public library path, not special-case internal code.

Files

• src/main.zig
• examples/
• README.md

Gates

• Gate A

Acceptance Criteria

• CLI commands exist for encode-tx, hash-tx, verify-sig, and rpc-check.
• Help text and examples are documented.
• CLI uses the same public library surface as consumers.

Related

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