Add experimental JTD to ESM validator code generator

[cursor]

Goal

Add an experimental CLI that reads a JTD schema (RFC 8927) and generates an ES2020 ESM validator exporting validate() for browser BroadcastChannel payload validation.

Scope (deliberately limited)

Support only:

• properties
• optionalProperties
• type primitives
• enum
• metadata.id

Reject the rest (elements, values, discriminator/mapping, ref/definitions, nested properties) with:

Unsupported JTD feature: <feature>. This experimental tool only supports flat schemas with properties, optionalProperties, type, and enum.

Deliverables

• New Maven module producing a shaded jtd-esm-codegen uber-jar (CLI entry point).
• Code in package io.github.simbo1905.json.jtd.codegen.
• Unit tests + sample schema.
• Nightly GitHub Actions workflow building jar + GraalVM native binaries for linux/windows/macos (amd64/arm64).