### Added
- NSIS plugin API (`pluginapi.cpp`/`pluginapi.h`) for 7-Zip zstd build
- `ExtractCallbackConsole` class for extraction progress and user input
- Main extraction logic (`Main.cpp`, `MainAr.cpp`) for the zstd bundle
- User input utilities (`UserInputUtils2`) for password handling and prompts
- Break signal handling (`NSISBreak`) for graceful interruption
- `Nsis7z_vs2026.vcxproj` (v145 toolset) for Visual Studio 2026 builds
- `tools/fix_vcxproj.py` helper for project file patching
- `versions/7-zip-zstd` submodule added
- `build_zstd.cmd` top-level build script for the zstd variant

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