feat: dispatch plot — stacked area of flow rates by carrier

[FBumann]

The headline energy-system view. Add result.plot.dispatch(carrier=...) returning a Plotly figure: stacked area of result.flow_rates for a carrier, signed via result.stats.carrier_balance (production up, consumption down), one trace per flow, with a net-balance line.

Acceptance: given a solved result, result.plot.dispatch("heat") renders a stacked area + net line, legend per flow.

Trivial — pure reduction over existing Result fields.