Change default serialization units

[kylecarow]

Built on #309

1. New canonical serialization units (proc macro: quantity_config)

The following quantities now serialize in practical engineering units instead of the SI base unit. The raw uom float is in the SI base, so each requires a new serialize_with helper to convert on write:

Quantity	Before	After
Energy	joules	kilojoules
Power	watts	kilowatts
Pressure	pascals	kilopascals
SpecificEnergy	joules/kg	kWh/kg
SpecificPower	W/kg	kW/kg
Temperature	kelvin	°C
TemperatureInterval	kelvin	°C

The Ratio "bare name legacy escape hatch" was also removed — Ratio fields now get the standard field_ratio / field_percent treatment like every other SI quantity.

2. New serialize_with helpers (serde_helpers.rs)

Seven new impl_si_serialize_as! expansions to support the above: energy_as_kilojoules, energy_as_kilowatt_hours, power_as_kilowatts, pressure_as_kilopascals, ratio_as_percent, specific_energy_as_kilowatt_hours_per_kilogram, specific_power_as_kilowatts_per_kilogram, temperature_as_degrees_celsius, temperature_interval_as_degrees_celsius.

3. Field-level #[si_unit(...)] overrides on struct fields

Across vehicle_model.rs, chassis.rs, conv.rs, hev.rs, hvac_*.rs, electric_machine.rs, fuel_converter.rs, fuel_storage.rs, transmission.rs, and reversible_energy_storage.rs — fields where the new global default isn't the right choice are overridden with an explicit #[si_unit(...)] annotation (e.g. #[si_unit(unitless)] for drag/rolling-resistance coefficients that should serialize as bare dimensionless ratios, #[si_unit(percent)] for SOC and efficiency fields).

Struct	Field	Type	#[si_unit]	Reason
Cycle	grade	Vec<si::Ratio>	percent	Grade is conventionally reported in %
CycleElement	grade	si::Ratio	percent	Matches Cycle.grade
Chassis	drag_coef	si::Ratio	unitless	Dimensionless aerodynamic coefficient (e.g. 0.3) — not a percentage
Chassis	wheel_rr_coef	si::Ratio	unitless	Rolling-resistance coefficient (e.g. 0.01) — dimensionless, not a percentage
Chassis	wheel_fric_coef	si::Ratio	unitless	Friction coefficient (e.g. 0.7) — dimensionless, not a percentage
ConventionalVehicle	alt_eff	si::Ratio	percent	Alternator efficiency, naturally expressed as %
RGWDBState	soc_fc_on_buffer	TrackedState<si::Ratio>	percent	SOC threshold expressed as %
RESGreedyWithDynamicBuffers	speed_soc_disch_buffer_coeff	Option<si::Ratio>	percent	SOC coefficient
RESGreedyWithDynamicBuffers	speed_soc_fc_on_buffer_coeff	Option<si::Ratio>	percent	SOC coefficient
RESGreedyWithDynamicBuffers	speed_soc_regen_buffer_coeff	Option<si::Ratio>	percent	SOC coefficient
HEVStopStartControl	soc_fc_forced_on	Option<si::Ratio>	percent	SOC threshold expressed as %
HVACSystemForLumpedCabinState	cop	TrackedState<Option<si::Ratio>>	unitless	Coefficient of performance (e.g. 2.5) — dimensionless ratio, not a percentage
HVACSystemForLumpedCabinAndRESState	cop	TrackedState<Option<si::Ratio>>	unitless	Same as above
ElectricMachineState	eff	TrackedState<si::Ratio>	percent	Efficiency
ElectricMachineState	eff_fwd_at_max_input	TrackedState<si::Ratio>	percent	Efficiency at peak input
ElectricMachineState	eff_at_max_regen	TrackedState<si::Ratio>	percent	Efficiency at max regen
FuelConverterState	eff	TrackedState<si::Ratio>	percent	Efficiency
FuelConverterThermalState	eff_coeff	TrackedState<si::Ratio>	percent	Thermal efficiency coefficient
FuelStorage	energy_capacity	si::Energy	kilowatt_hours	Overrides the new global Energy default (kJ) — kWh is more natural for fuel
ReversibleEnergyStorage	energy_capacity	si::Energy	kilowatt_hours	Battery capacity in kWh is universal convention
ReversibleEnergyStorage	min_soc	si::Ratio	percent	SOC limit
ReversibleEnergyStorage	max_soc	si::Ratio	percent	SOC limit
ReversibleEnergyStorageState	soc	TrackedState<si::Ratio>	percent	State of charge
ReversibleEnergyStorageState	soc_regen_buffer	TrackedState<si::Ratio>	percent	SOC buffer threshold
ReversibleEnergyStorageState	soc_disch_buffer	TrackedState<si::Ratio>	percent	SOC buffer threshold
ReversibleEnergyStorageState	eff	TrackedState<si::Ratio>	percent	Efficiency
ReversibleEnergyStorageState	soh	TrackedState<si::Ratio>	percent	State of health (type also promoted from TrackedState<f64>)
TransmissionState	eff	TrackedState<si::Ratio>	percent	Efficiency
VehicleState	grade_curr	TrackedState<si::Ratio>	percent	Current road grade

The core distinction between unitless and percent is physical meaning: dimensionless coefficients used in multiplication (drag, rolling resistance, friction, COP) serialize with their raw value and get unitless; fractions that represent a proportion of a maximum (efficiency, SOC, grade) serialize as percentages.

4. soh field promoted to si::Ratio

In ReversibleEnergyStorage, soh was a raw TrackedState<f64> — it's now TrackedState<si::Ratio> with #[si_unit(percent)], giving it the same dimensional-safety and multi-unit deserialization that all other SI fields have.

5. Integration test updates

tests/test_serde_api_integration.rs updated to match the new canonical key names (e.g. energy_capacity_kilowatt_hours instead of energy_capacity_joules, min_soc_percent instead of min_soc_ratio).

TODO: perhaps fields like friction/drag/rr coefficients should never have been ratios?