add java end-to-end study example

[avinxshKD]

This adds a runnable Java end-to-end study example in the standard concore file-based loop, plus short run instructions and a lightweight smoke check.
This makes Java usage easy to validate in a real study path.
closes #522

[avinxshKD]

Hey @pradeeban @Rahuljagwani @mayureshkothare, pls take a look, Kept it intentionally narrow so it’s useful now without dragging in runtime or protocol changes, happy to do generator-flow integration as a follow-up if you want that next.

Java support is much easier to verify in practice now

[avinxshKD]

@pradeeban Hope Java looks much better now :p

I genuinely enjoyed implementing this and I’m really happy with how it turned out tbh.

I’ll take on extending conformance matrix coverage beyond the current parser/API cases next.