Plan private gallery web engine

[PrzemyslawKlys]

Summary

• add the PowerForge.Web private gallery engine plan as a dedicated architecture document
• define library boundaries for PowerForge, PowerForge.Web, PowerForge.Web.Cli, PSPublishModule, and PowerForge.PowerShell
• cross-link the private gallery indexing lane from the web roadmap and library ecosystem plan

Validation

• git diff --check -- Docs/PowerForge.Web.PrivateGallery.md Docs/PowerForge.Web.Roadmap.md Docs/PowerForge.Web.LibraryEcosystemPlan.md

Stacking

• stacked on top of Add private gallery publish process #390 / codex/private-gallery-publish-flow