Add battery energy integration and knockdown factors

[karakayahuseyin]

Extend RateMapBattery with bat-perf style mission/endurance utilities.

Current implementation covers point-state behavior well:

• voltage sag: V = OCV(dod) - R(dod) * I
• current / C-rate / voltage / power / load-resistance states
• pack series/parallel scaling
• propulsion solver integration

Missing bat-perf-style pieces:

• integrate battery state over time for constant current and constant power
• stop integration at cutoff voltage or DOD limits
• report delivered energy, consumed charge, final state, and stop reason
• add usable-energy knockdown factors similar to kedod and kemfg

Why:

Simple Coulomb counting can overestimate endurance at high C-rate. Voltage sag can hit cutoff before nominal capacity is fully used.

Acceptance criteria:

• Add integrate_current(...) and integrate_power(...)
• Add delivered energy / consumed Ah outputs
• Add cutoff and DOD-bound stop reasons
• Add tests for high-load early cutoff
• Add one example showing usable energy vs simple SoC counting