docs(wpf-interop): replace speculative surface with stub + workaround (#335)

* docs(wpf-interop): replace speculative surface with stub + workaround

The page previously described a hypothetical Reactor.Interop.Wpf
surface — bootstrap, host control, reference table, screenshots — that
hasn't shipped yet. Readers had to wade through ~190 lines of
roadmap-tense prose to learn the actually-actionable bit:
DesktopWindowXamlSource works today.

Demote to tier=stub. Keep just two paragraphs: a "coming soon" status
line and the WinAppSDK-primitive workaround, with Next Steps pointing
at WinForms interop as the shape reference.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* docs(wpf-interop): remove orphaned doc app

The wpf-interop doc app exposed snippets (bootstrap, host-element,
data-flow, threading, accessibility) and a screenshot (host-element)
that the previous commit stopped referencing when the template was
demoted to a stub. Compile is clean without them; remove the dead
weight.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: codemonkeychris