Add tuple-form union variants

[abdushakoor12]

TLDR

Add tuple-form union variants like Number(Int) to the DSL, render them as tuple variants in Rust output, and document the new syntax with examples.

Details

Closes #24.

What Was Added

• Tuple-form union variant syntax in the parser and lexer via ( / ) support.
• Rust converter support for emitting tuple enum variants such as Number(i64).
• A tuple-variant regression test covering union RequestId { Number(Int) String(String) }.
• Language-reference and VS Code sample examples for tuple-form variants.

What Was Changed/Deleted

• Union printing and layout now preserve tuple-form variants as Variant(T) instead of rendering synthetic field names like _0: T.
• The typediagram core bundle-size budget was raised from 75 KB to 75.5 KB to account for the new syntax support.

Breaking Changes

• No breaking changes.

How Do The Automated Tests Prove It Works?

• packages/typediagram/test/converters/rust.test.ts now includes emits tuple enum variants from tuple-form union syntax, which parses tuple-form DSL and asserts Rust output contains Number(i64) and String(String).
• make ci passed for the branch, including typediagram, CLI, VS Code, and web test suites, web Playwright coverage merge, and the typediagram bundle-size gate.