Bring `flepimop2-op_system` adapter under mypy --strict

[jc-macdonald]

Summary

Remove the flepimop2-op_system/ exclusion from pyproject.toml mypy config and fix all resulting strict-mode errors. Add a py.typed marker to the provider package.

Problem

pyproject.toml explicitly excludes the adapter:

[tool.mypy]
strict = true
exclude = ["examples/", "flepimop2-op_system/"]

This means type errors in the provider go undetected. The op_engine provider just went through this same cleanup (PR #51, op_engine commit 25d3dbc) — StateChangeEnum vs string literals, dict vs Pydantic model constructors, narrowing type: ignore annotations.

The op_system adapter has the same patterns:

• _AxesMeta is a bare NamedTuple (could be typed more strictly)
• Untyped getattr calls
• No py.typed marker for downstream type checking

Scope

• Add flepimop2-op_system/src/flepimop2/system/op_system/py.typed
• Remove "flepimop2-op_system/" from mypy exclude
• Fix all mypy --strict errors that surface
• Narrow any remaining type: ignore to specific error codes

Related

• op_engine PR Fix provider package mypy error: StateChangeEnum not found in flepimop2.typing #51 (same cleanup for op_engine provider)
• Replace system._stepper access with public bind API #55 (_bind_impl — PR Implement _bind_impl for SystemABC compliance #57)