Add `py.typed` marker to `op_engine` core package

[jc-macdonald]

Summary

Add a py.typed marker file to src/op_engine/ so downstream consumers (including the flepimop2 provider) get full mypy type checking.

Context

PR #51 added py.typed to the flepimop2 provider package (flepimop2-op_engine), but the core op_engine package still lacks it. Without the marker, mypy skips type analysis of op_engine imports with --strict.

Scope

• Add empty src/op_engine/py.typed
• Ensure pyproject.toml includes py.typed in package data
• Fix any mypy errors that surface once the marker is present

Related

• PR Expose all CoreSolver methods, extend validate_system(), wire jacobian #51 (added py.typed to provider)

[jc-macdonald]

Closing as completed: src/op_engine/py.typed is present and pyproject includes the marker in wheel packaging.