Document shared WinForms/WPF Clipboard/OLE architecture and cross-repo change risk

[Copilot]

This PR adds focused documentation for the shared Clipboard/OLE implementation used by WinForms and WPF, including how shared bits flow through transport packaging and where cross-repo regressions can originate. It also clarifies operational tradeoffs and change-management expectations for contributors touching shared code.

• Shared architecture and transport path

  • Clarifies the shared implementation surface in System.Private.Windows.Core (especially Ole/).
  • Documents how bits move via Microsoft.Private.Winforms transport package into Windows Desktop packs consumed by both UI stacks.
  • Makes runtime implication explicit: both frameworks execute the same System.Private.Windows.Core clipboard/OLE behavior.

• Case studies and impact traceability

  • Adds an explicit customer-facing case study for #14308 (WPF clipboard CFG crash scenario) and links the remediation path through #14257 (main) and #14296 (servicing backport).
  • Keeps existing technical case studies and reorders them into a clearer narrative from user-visible regression → root-cause classes.

• Change-management guidance and tradeoffs

  • Expands liabilities with explicit ownership ambiguity (code hosted in dotnet/winforms, behavior shared by WinForms/WPF callers).
  • Reinforces review/validation expectations for cross-stack safety, especially in clipboard/OLE servicing scenarios.

### Case Study 1: Customer-visible CFG Crash in WPF Clipboard Calls (WinForms #14308)

**Cross-repo impact path:**
- Reported as WPF scenario in WinForms issue #14308
- Root cause in shared `System.Private.Windows.Core` clipboard/OLE code
- Fix in WinForms PR #14257, backport in #14296

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• v53vsblobprodcus320.vsblob.vsassets.io
  • Triggering command: /home/REDACTED/work/winforms/winforms/.dotnet/dotnet /home/REDACTED/work/winforms/winforms/.dotnet/dotnet package download Microsoft.DotNet.Arcade.Sdk@11.0.0-beta.26269.108 --verbosity minimal --prerelease --output /home/REDACTED/.nuget/packages/ --configfile /home/REDACTED/work/winforms/winforms/NuGet.config (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

[merriemcgaw]

@Shyam-Gupta can you review some of this new documentation?