edit connector class

[MacdonaldJoshuaCaleb]

This pull request introduces improvements to the operator wiring and kernel parameter handling in the op_engine, as well as enhancements to the test suite and developer tooling. The main changes include merging precomputed mixing kernels into the right-hand side (RHS) parameters, initial support for passing operator specifications (especially for IMEX methods), and updates to the development workflow to better separate provider and core tasks.

Core engine improvements:

• The engine now merges any precomputed mixing_kernels exposed by a system into the RHS parameters, allowing systems to inject kernel values directly into the computation.
• Initial wiring for operator specifications: if operator specs are provided in the config (notably for IMEX methods), the default operator is passed to the solver, enabling identity operator tests and future extensibility.

Testing enhancements:

• Added _KernelStepper, _KernelSystem, and _ImexSystem test helpers, along with new tests to verify that mixing kernels are passed correctly and that IMEX operator specs are accepted and functional.
  Developer tooling and workflow:

• Updated the justfile to better separate provider and core tasks, add "nosync" variants for faster CI, and use uvx for tool-only tasks. The CI aggregate step is now more efficient and explicit.

Minor cleanup:

• Removed stale contract comments from reshape_for_axis_solve in model_core.py.

Closes #8 in prep for adding multiple demos to flepi demo.

[pearsonca]

Minor items; going to try it out w/ flepimop-demo

[TimothyWillard]

Updated the justfile to better separate provider and core tasks, add "nosync" variants for faster CI, and use uvx for tool-only tasks. The CI aggregate step is now more efficient and explicit.

Perhaps these changes would be better suited for a separate PR. The failing CI is making it a bit annoying to verify the the actual code changes pass quality checks. Also, I do not think the premise for this change is true. uvx runs tools in an isolated virtual environment so:

1. You loose the efficiency gains from the pyproject.toml caching mechanism from https://github.com/astral-sh/setup-uv.
2. No longer able to manage dev dependencies via the pyproject.toml (i.e. restrict or require versions).

[TimothyWillard]

Minor code edits, larger issues about the CI changes. Is there any documentation for these mixing kernels? I do not really understand what they are or how to use them?

[MacdonaldJoshuaCaleb]

Addressed reviewer comments including reverting the just file and opened new issues regarding Operator handling (#11) as well as an issue for handoff and expansion of flepimop2 SystemABC objects (flepimop2 #131)