feat(sim): add core foundation with SimConfig, Engine, Controller, KPI

[bugsbunny88]

Summary

• Rewrites simulation architecture around a Device trait, SimConfig, Engine, and NaiveRtController
• KpiReport tracks energy, cost, and self-sufficiency metrics
• Feeder model enforces import/export limits; power balance follows sign convention (+kW=load, -kW=gen)
• Integration tests cover energy conservation, sign invariants, and determinism

Stacked PR series

This is the first of a 6-part stacked series. Merge order:

1. feat(sim): add core foundation with SimConfig, Engine, Controller, KPI #55 - Core foundation (this PR)
2. feat(sim): add stochastic weather, CSV export, imbalance settlement #56 - Stochastic weather, CSV export, imbalance settlement
3. feat(app): add CLI with TOML config and scenario presets #57 - CLI with TOML config and scenario presets
4. feat(sim): add GreedyController with forecast-aware battery dispatch #58 - GreedyController with forecast-aware battery dispatch
5. feat(app): add REST API behind api feature gate #59 - REST API (feature-gated)
6. feat(app): add live TUI behind tui feature gate #60 - Live TUI (feature-gated)

Each PR builds on the previous one. Once #55 is merged, #56's diff will automatically shrink to just its own changes, and so on.

To see the complete end state with all 6 PRs combined, browse the preview/complete branch on my fork.

Test plan

• cargo test passes (69 unit + 7 integration + 2 doc-tests)
• cargo check clean