[FEATURE] Q-limit active-set handling

[figueroa1395]

Describe the feature request

This issue aims to describe a voltage-setpoint controller-like mechanism for the Q limit handling in PV nodes.

Description taken from #1236 (comment):

our proposal is not to extend the Newton-Raphson equations. The idea is to handle this in the same active-set loop that already performs PV→PQ switching: before switching a machine-connected PV node to PQ, the solver may first try to adjust the generator voltage setpoint in configured discrete steps.

These steps can represent either generator excitation adjustment or a machine-transformer-related voltage step. The proposal is intentionally not meant as a generic transformer tap-control mechanism, and the machine transformer should not be controlled together with a regular transformer tap changer. From our perspective, the machine transformer is a special case: it adapts the generator connection to the grid voltage level, while a normal tap changer typically controls transformers between voltage levels.

The proposed additional parameters would therefore be attached to the machine/generator-related control path, for example:

• voltage step size, e.g. delta_u per step,
• minimum and maximum allowed step,
• optionally a voltage deviation limit beyond which the adjustment is no longer used.

If a Q-limit violation occurs, the solver would first try to move the voltage setpoint within this configured range so that PV operation can be maintained. If the adjustment range is exhausted, or if the voltage would deviate too far, the existing PV→PQ fallback is used.

So conceptually this is still part of the Q-limit active-set handling, not an extension of the NR equation system and not a generic outer-loop tap-position optimizer.

Relates to #185 and #1236, specifically to #1236 (comment) and #1236 (comment)

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