SFT phase of TwinRL

[int-smart]

Title

TwinRL implementation

Summary / Motivation

• PR has SFT/offline phase based on TwinRL Jax code.

Related issues

• Fixes / Closes: # (if any)
• Related: # (if any)

What changed

• Short, concrete bullets explaining the functional changes (how the behavior or output differs now).
• Short note if this introduces breaking changes and migration steps.

How was this tested (or how to run locally)

• Tests added: list new tests or test files. pytest -q tests/ -k <keyword>
• Manual checks / dataset runs performed.
• Instructions for the reviewer for reproducing with a quick example or CLI (if applicable)

Checklist (required before merge)

• Linting/formatting run (pre-commit run -a)
• All tests pass locally (pytest)
• Documentation updated
• CI is green
• Community Review: I have reviewed another contributor's open PR and linked it here: # (insert PR number/link)

Reviewer notes

• Anything the reviewer should focus on (performance, edge-cases, specific files) or general notes.
• Anyone in the community is free to review the PR.