fix(smart): correct inverted v.b.power sign on SQ/SE (issue #40)

[kezarjg]

What & why

Fixes the inverted battery-power sign on smart 453/forfour (SQ) and smart ED/fortwo (SE), which causes ABRP's calibrated reference consumption to drift toward zero (#40).

Both OVMS smart modules report v.b.power with the wrong sign - negative while driving (consuming), positive while charging - the opposite of the OVMS core / Iternio convention (positive = output/discharging). ABRP uses power for consumption calibration, so it reads consumption as regen and the reference consumption decays.

This was confirmed with on-device captures from both cars (telemetry as received by ABRP):

	forfour (SQ), n=1747	fortwo (SE), n=431
median( power / (V*I) )	-0.999	-1.000
driving (>= 5 m/s): current > 0	90%	86%
driving (>= 5 m/s): power > 0	7%	10%

Driving is net discharge (must be positive by convention), so v.b.current is the correctly-signed metric and v.b.power is inverted on both cars.

The fix

overrideMetricMap now corrects power for SQ/SE by keeping the module's power magnitude and taking its sign from v.b.current:

if (current < 0) { return -Math.abs(power); }
if (current > 0) { return Math.abs(power); }
return power; // current == 0: power ~0, sign irrelevant

Self-healing: v.b.current is correctly signed in both the current (buggy) firmware and a future upstream-fixed firmware (the fix only touches v.b.power). So once the module's v.b.power sign is corrected upstream, this override returns v.b.power unchanged - it becomes a no-op with no plugin change needed to retire it.

Scope is limited to power; current, is_charging, and is_dcfc are unchanged.

Upstream

The root defect is in the OVMS vehicle-module firmware; separate firmware bugs are being filed against vehicle_smarteq and vehicle_smarted. This PR is the plugin-side workaround so affected users get correct calibration now.

Tests

Added overrideMetricMap coverage for the smart override (driving correction, charging correction, post-fix no-op, zero-current, SE shares the override, unsupported when current absent, and non-smart passthrough). Full suite: 71 passing. VERSION -> 2.3.1, CHANGELOG updated.