Rivet 0.10.0

• Host platforms ship as linkable packages — downstream products never
  compile Rivet sources into their own build lists (constraint recorded
  in #238). Linux gains a rivet::platform static library under
  platform/linux/ that the host template, the integration binary, and
  the taskboard example add_subdirectory() and link; the scaffold's
  generated CMake does the same. Windows gains
  platform/windows/rivet-host.vcxproj (static library) which the host
  template references via ProjectReference, with hybrid-CRT hosts
  forwarding their RuntimeLibrary choice through
  RivetRuntimeLibrary. A root Package.swift exposes the macOS
  products as a tag-resolvable SwiftPM package
  (.package(url: …, from: "0.10.0")) beside the in-repo path package.
  The WinUI HWND bridge moves to winui_window.hpp so the Windows
  library compiles without Windows App SDK include paths.