Set up `oxa-types` Rust crates.io package

[nokome]

Set up the Rust package for OXA types using typify for code generation from JSON Schema. Create packages/oxa-types-rs/ with:

Cargo.toml

• Package name: oxa-types
• Dependencies: serde, serde_json

src/lib.rs (generated)

• Rust types generated from JSON Schema using typify
• Proper serde attributes for JSON serialization
• Tagged enums for Block/Inline union types

Codegen integration

• Implement scripts/lib/generate-rs.ts to invoke the generate-rs binary
• The scripts/lib/generate-rs.rs binary uses typify to generate Rust code. Avoid having Rust as a hard dependency (skip if not available)
• Register in scripts/codegen.ts generators array
• Add pnpm codegen rs command

Tasks

• Create packages/oxa-types-rs/Cargo.toml
• Create placeholder packages/oxa-types-rs/src/lib.rs
• Implement scripts/lib/generate-rs.ts to invoke Rust binary
• Complete scripts/lib/generate-rs.rs typify implementation
• Register Rust generator in scripts/codegen.ts
• Run pnpm codegen rs and verify generated types
• Verify cargo build succeeds for the package