Move validation logic from CoreSolver to RunConfig/RunPlan

[jc-macdonald]

Validation of solver configuration is currently embedded in CoreSolver methods, but belongs in RunConfig/RunPlan dataclasses using __post_init__.

Problem

CoreSolver performs input validation that should happen at configuration time, not at solve time. This makes the solver harder to test and blurs responsibility boundaries.

Scope

• Move validation at L840 and L855 of core_solver.py into RunConfig/RunPlan.__post_init__
• Ensure errors are raised at config construction time with clear messages
• Update tests accordingly

Related

• Resolve CoreSolver mutable+return state pattern #32 (CoreSolver mutable-state cleanup)

Ref: PR #18 review by @TimothyWillard (L840, L855)