Prevent GDI+ startup from blocking Windows shell

[coneilen]

Summary

Prevent optional GDI+ initialization from blocking the Windows UI thread before
the native shell window is created. The beta5 Dev Box evidence narrowed the
90-second no-window hang to synchronous GdiplusStartup; plain GDI remains the
fallback until asynchronous initialization succeeds.

Changes

• Start process-global GDI+ initialization on a detached worker instead of the
  UI thread.
• Publish GDI+ availability and coordinate draw sessions/shutdown under one
  mutex, preventing data races and shutdown during active drawing.
• Keep GDI+ disabled for the existing daemon-handoff and UIA hooks.
• Add a child-process regression whose startup provider deliberately blocks,
  while the process uses exactly the supported 11-key environment and must
  still expose a native window within a bounded deadline.
• Register the live regression in the Windows shell suite with explicit
  exited/no-window/window and zero-observation outcomes.

Test plan

RED: pwsh -NoProfile -File Tools\windows\Tests\GdiplusStartup.Live.Tests.ps1 with pinned Zig and Winghostty -> process remained alive but no native window appeared; observations=80; exact 11 keys
GREEN: pwsh -NoProfile -File Tools\windows\Tests\GdiplusStartup.Live.Tests.ps1 with pinned Zig and Winghostty -> PASS; executed=1; observations=2; native window appeared
REGRESSION: pwsh -NoProfile -File Tools\windows\validate.ps1 -Task windows-shell -SkipTrayLive -ShellValidationRoot C:\v7 -> PASS; Windows shell 53/53, App 739/739, scrubbed startup 4/4

• zig test src\GdiplusAA.zig with pinned Zig: 1/1 passed.
• Tools\windows\Tests\ValidationRunner.Tests.ps1: passed, including 380
  multi-project protocol contracts.
• The real source-built shell passed all four exact-environment onboarding
  cases with developer-tool paths excluded.
• Microsoft documents that GdiplusStartup and GdiplusShutdown may run on
  different threads when GDI+ objects are gone before shutdown. Shutdown is
  therefore invoked only after the message loop, while the mutex prevents
  overlap with active draw sessions.
• A rebuilt candidate and attended Dev Box rerun are still required. No tag,
  package, release, backend, Worktrees, Nod, dump, or publication action is
  included here.
• macOS make test and make check were not run because this change is scoped
  to the Windows Zig shell and Windows validation.

Checklist

• I have read the Contributing Guidelines
• I have signed off my commits (git commit -s) per the DCO
• Tests pass locally (make test)
• Code follows the existing style (make check)
• I added the test/contract before the implementation and observed the intended RED failure