Battery backend: expose accuracy knobs via stable config surface

[cmccomb]

Summary

Keep the existing high-level problem APIs stable while exposing battery-backend fidelity controls through a clean config surface.

Scope

• Add/standardize config surface via battery problem TOML and/or a BatteryBackendConfig dataclass:
  • cell_model_mode
  • parameter_set
  • thermal_mode
  • ambient_temp_C
  • parasitics (R_bus, R_contact, etc.)
  • solver tolerances/timestep policy
• Ensure grammar and optimization battery problems share this surface consistently
• Document defaults and migration notes for v0.2.x users

Acceptance criteria

• Existing battery examples still run without required user changes
• New knobs are discoverable in docs and problem metadata
• Serialization/deserialization of config is tested

[cmccomb]

Completed on battery-tuneup before/within PR #55. The shared BatteryBackendConfig surface now carries cell_model_mode, parameterization, thermal_mode, ambient_temp_c, parasitics, and solver_policy, is parsed from battery problem TOML, and is exercised by existing battery problem/evaluator tests while preserving backward-compatible examples.