Skill for UnitsNet v6

[lipchev]

Given the large number of changes I though it might be useful to create something like a migration skill so that agents don't conflate concepts from the v5. The one I've created for myself has a lot of repo-specific extras, so it isn't very useful as a whole, but here are a few general purpose tips from it:

Treat the installed API as UnitsNet v6, not the more familiar v5 API. Verify the pinned package version in the target project before copying patterns from other repositories or general model knowledge.

• Use QuantityValue for the numeric component of quantity calculations. In v6 it is the evolved representation returned by .Value and unit-specific value properties; do not casually narrow it to double.
• Keep double only at verified persistence, CSV, JSON, charting, or external API boundaries that require it. Convert at that boundary, not throughout the domain calculation.
• Expect .Value, converted unit properties, arithmetic helpers, and many factory overloads to use QuantityValue in v6.
• Use QuantityValue.Zero, One, PositiveInfinity, Abs, Sqrt, and Pow where quantity-domain calculations require them.

Also there should probably be something regarding the string formats and possibly some performance tips (avoiding boxing etc.).