Add Camera.forward() world-space viewing direction

[AshSgDe29071999]

Summary

• Adds Camera.forward() returning the world-space unit viewing direction (OpenCV +z / third row of R).
• Adds a look-at round-trip test asserting unit length and normalize(target - eye).

Fixes #58

Test plan

• Forward vector matches look-at direction and has unit norm

[bamdadd]

Thanks @AshSgDe29071999, and welcome! You and another contributor landed on the same issue (#58) at the same time — #61 came in just ahead and is type-clean under our mypy-strict gate, so I've merged that one. Your implementation was correct on the math; the only gap was mypy (Returning Any from self.rotation()[2].copy()). Really appreciate the PR — please grab another open issue (there are fresh good-first-issues on the tracker, e.g. #59-style validators) and I'll get you a merge. Closing this as a duplicate of #61, no reflection on the work.