Remove StateDict type and ensure_sim_state helper

[orionarcher]

Orion Summary

state: SimState | StateDict is a longstanding compromise in the function signature. As we add more robust typing, I think it's time to fully embrace SimState as the core object. This PR swaps from state: SimState | StateDict to state: SimState across the board. (already got agreement from @abhijeetgangan)

Upstream Nequip issue fixed here. Fairchem issue is a flake.

Claude Summary

• Removes StateDict = dict type alias and StateKey from torch_sim/typing.py — these were the only remaining artifacts of the dict-as-state pattern
• Removes the ensure_sim_state conversion helper from torch_sim/state.py — it's now a pure no-op since all callers pass a SimState
• Simplifies state: SimState | StateDict → state: SimState across all integrators (NVE, NVT, NPT), optimizers (FIRE, BFGS, L-BFGS, gradient descent), models (MACE, ORB, SevenNet, FairChem, FairChem legacy, soft sphere, GraphPES, Morse, LJ, particle life, metatomic, MatterSim), and the ModelInterface ABC
• Updates two test functions that explicitly tested dict-as-state to pass SimState directly instead
• Updates the introduction example script to construct ts.SimState(...) instead of raw dict(...)

Test plan

• All pre-commit hooks pass (ruff, codespell, ty)
• grep -r "StateDict|ensure_sim_state" torch_sim/ tests/ returns no matches
• Full test suite passes: 656 passed, 3 skipped

🤖 Generated with Claude Code

[CompRhys]

this is a pain point of moving to externalize, the upstream nequip has a dep on StateDict which causes the tests to fail.

This will impact Nequip, SevenNet and Orb (@cw-tan, @YutackPark, @vsimkus) which have already been externalized. Sorry.