Add Metalama 2026.1 and separate MetalamaVsx product family

Create Metalama 2026.1 product family depending on PostSharp 2026.0. Move Metalama.Vsx to its own independent product family (2026.0 only) to break the merge chain from 2025.1 versions. Remove obsolete 2025.0 versions for both Metalama and PostSharp.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: gfraiteur