refactor: rename versions/zstd to versions/zstd-bundle for consistency

Aligns the zstd NSIS wrapper directory with the naming convention used by
all other bundle directories (versions/25.01-bundle, 26.00-bundle, etc.).

Changes:
- git mv versions/zstd -> versions/zstd-bundle (20 project-owned files)
- Fix UI/NSIS/StdAfx.h: use relative include ../../Bundles/Nsis7z/pluginapi.h
  instead of absolute-style 7zip/Bundles/Nsis7z/pluginapi.h (portable across
  both Linux and Windows builds without extra include path)

Build tool updates:
- tools/legacy/build_plugin_zstd_vs2026.py: updated project_dir path
- tools/linux/build_plugin_linux.py:
  * VERSION_LAYOUT zstd: zstd -> zstd-bundle
  * VERSION_EXTRA_FLAGS zstd: -Wno-sign-compare -Wno-cast-function-type
    (fast-lzma2 C vendor code)
  * VERSION_EXTRA_CXXFLAGS zstd: -Wno-unused-parameter (NSIS wrapper)
  * EXTRA_SYS_OBJS: inject MyWindows.o for zstd (LocalFileTimeToFileTime2)
  * Remove 'zip_version != zstd' exception; auto-recreate symlinks for all
    bundle versions including zstd
- tools/linux/setup_bundle_symlinks.py: add VENDOR_DIR mapping so 'zstd'
  points to versions/7-zip-zstd (not versions/zstd which no longer exists)
- tools/linux/overlay/makefile.gcc: add EXTRA_SYS_OBJS ?= variable so
  callers can inject additional objects without modifying the file

Linux build status after this commit:
  python3 build_plugin.py --7zip-version zstd  -> Build completed (all 3 configs)

tools/legacy/build_plugin_zstd_vs2026.py

@@ -5,7 +5,7 @@ Supports multiple build configurations with flexible parameters.
 Uses PlatformToolset v145 (Visual Studio 2026 Build Tools).
 
 The 7-Zip ZS source is pulled from the git submodule at versions/7-zip-zstd/.
-The NSIS plugin wrapper (nsis7z.cpp, vcxproj, …) lives in versions/zstd/.
+The NSIS plugin wrapper (nsis7z.cpp, vcxproj, …) lives in versions/zstd-bundle/.
 
 NOTE: VS2026 Build Tools use v145 toolset (version 18.x)
 """
@@ -201,7 +201,7 @@ def find_msbuild(vs_version: str = 'auto') -> 'Optional[Tuple[Path, str, str]]':
 def get_project_paths(toolset: str = 'v145') -> Tuple[Path, Path, Path]:
     """Get project directory, project file, and plugins directory"""
     script_dir = Path(__file__).parent.absolute()
-    project_dir = script_dir.parent.parent / 'versions' / 'zstd'
+    project_dir = script_dir.parent.parent / 'versions' / 'zstd-bundle'
     vcxproj = 'Nsis7z_vs2026.vcxproj'
     project_file = project_dir / 'CPP' / '7zip' / 'Bundles' / 'Nsis7z' / vcxproj
     plugins_dir = script_dir.parent.parent / 'plugins'