submodule: add versions/26.01 (ip7z/7zip tag 26.01)
Registers the upstream ip7z/7zip repository as a git submodule at versions/26.01, checked out at tag 26.01 (commit 8c63d71). This provides the 7-Zip 26.01 vendor source tree for both the Windows (MSBuild/vcxproj) and Linux (MinGW cross-compilation) builds.
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