Add challenge: Parallel Reverse Scan (GAE)

[basilwong]

Summary

• Add the hard Parallel Reverse Scan (GAE) challenge.
• Include reference implementation, tests, HTML specification, and six framework starters.
• Validate the JAX implementation on a Modal T4.

Important Note

Challenge number 28 in the code is provisional and may be changed before merging to avoid conflicts with existing or pending challenges.

Validation

• Black and Flake8 pass.
• Structural and numerical checks pass locally.
• Modal T4 runtime validation to be performed against the JAX implementation.

Online HTML Viewer:

Testing:
https://modal.com/notebooks/basilwong/main/nb-R5w7UJTS6jCNJRGozAssnN