Feat: SoC balance Cycle option with configurable hysteresis

[wouterbouvy]

Summary

• Adds a new SoC balance Cycle option that prioritizes the lowest-SoC battery when charging and the highest-SoC battery when discharging on every control cycle.
• Includes configurable SoC balance hysteresis (default 3%) so priority does not chatter when SoCs are close; set to 0 for pure SoC sort.
• Especially helpful for mixed-capacity setups (different kWh). Existing Auto balance (30-minute round-robin) is unchanged.

Test plan

• Reload HA helpers / package and confirm Cycle shows SoC balance and the new hysteresis helper (default 3%)
• Select SoC balance with 2+ batteries at different SoC; verify charge prefers lowest SoC and discharge prefers highest SoC
• With hysteresis at 3%, confirm priority does not flip when SoC gap is < 3%
• Set hysteresis to 0 and confirm order follows live SoC every cycle
• Confirm Auto balance / Daily / Weekly / Never still behave as before
• Run ./contribute/check.ps1 and confirm it passes

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