feat(26.01): add bundle wrapper directory with NSIS sources and vendor symlinks

7-Zip 26.01 (ip7z/7zip) never shipped NSIS plugin sources, so they live in
a separate bundle directory (versions/26.01-bundle/) that mirrors the layout
expected by the shared overlay makefile and by MSBuild.

Bundle contents:
- CPP/7zip/Bundles/Nsis7z/: NSIS plugin wrapper sources (nsis7z.cpp,
  pluginapi.cpp, api.h, resource.rc, StdAfx.h, …) and the Windows build
  project file (Nsis7z_vs2026.vcxproj, adapted from versions/26.00/).
  The vcxproj references the vendor submodule tree via relative paths
  (../../../../../26.01/…), keeping the submodule pristine.
- CPP/7zip/UI/NSIS/: NSIS UI adapter sources (Main.cpp, MainAr.cpp,
  NSISBreak.cpp, ExtractCallbackConsole.cpp, UserInputUtils2.cpp and
  corresponding headers), copied from versions/26.00/.
- CPP/7zip/LzmaDec_gcc.mak: forwarding stub; GNU make resolves relative
  `include` paths against the CWD, so when make runs from Bundles/Nsis7z
  the upstream Arc_gcc.mak's `include ../../LzmaDec_gcc.mak` lands here.
  The stub re-includes the real file via $(VENDOR_7ZIP).
- Symlinks (C, Asm, CPP/7zip/*, CPP/Common, CPP/Windows, …) pointing into
  the versions/26.01 submodule.  These are required because the vendor
  makefile uses CWD-relative source paths (../../../../C/…, ../../Common/…).
  tools/linux/setup_26_01_bundle_symlinks.py recreates them if needed.

versions/26.01-bundle/CPP/7zip/warn_gcc.mak

@@ -0,0 +1 @@
+../../../26.01/CPP/7zip/warn_gcc.mak