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)

versions/zstd-bundle/CPP/7zip/UI/NSIS/ExtractCallbackConsole.h

@@ -0,0 +1,98 @@
+// ExtractCallbackConsole.h
+// NSIS version adapted for 7-Zip 25.01 API
+
+#ifndef __EXTRACTCALLBACKCONSOLE_H
+#define __EXTRACTCALLBACKCONSOLE_H
+
+#include "Common/MyCom.h"
+#include "Common/MyString.h"
+#include "Common/StdOutStream.h"
+#include "7zip/Common/FileStreams.h"
+#include "7zip/IPassword.h"
+#include "7zip/Archive/IArchive.h"
+#include "7zip/UI/Common/ArchiveExtractCallback.h"
+#include "7zip/UI/Console/OpenCallbackConsole.h"
+
+typedef void (*ExtractProgressHandler)(UInt64 completedSize, UInt64 totalSize);
+typedef void (*ExtractProgressWithFileHandler)(UInt64 completedSize, UInt64 totalSize, const wchar_t *fileName);
+
+class CExtractCallbackConsole Z7_final:
+  public IFolderArchiveExtractCallback,
+  public IExtractCallbackUI,
+#ifndef Z7_NO_CRYPTO
+  public ICryptoGetTextPassword,
+#endif
+  public COpenCallbackConsole,
+  public CMyUnknownImp
+{
+  Z7_COM_QI_BEGIN2(IFolderArchiveExtractCallback)
+#ifndef Z7_NO_CRYPTO
+  Z7_COM_QI_ENTRY(ICryptoGetTextPassword)
+#endif
+  Z7_COM_QI_END
+  Z7_COM_ADDREF_RELEASE
+
+  // IProgress
+  Z7_IFACE_COM7_IMP(IProgress)
+  
+  // IFolderArchiveExtractCallback
+  Z7_IFACE_COM7_IMP(IFolderArchiveExtractCallback)
+
+  // IExtractCallbackUI (non-COM interface)
+  Z7_IFACE_IMP(IExtractCallbackUI)
+
+#ifndef Z7_NO_CRYPTO
+  Z7_IFACE_COM7_IMP(ICryptoGetTextPassword)
+#endif
+
+public:
+#ifndef Z7_NO_CRYPTO
+  bool PasswordIsDefined;
+  UString Password;
+#endif
+  
+  UInt64 NumArchives;
+  UInt64 NumArchiveErrors;
+  UInt64 NumFileErrors;
+  UInt64 NumFileErrorsInCurrentArchive;
+
+  CStdOutStream *OutStream;
+
+  UInt64 totalSize, completedSize, lastVal;
+
+  ExtractProgressHandler ProgressHandler;
+  ExtractProgressWithFileHandler ProgressWithFileHandler;
+  UString CurrentFileName;
+
+  CExtractCallbackConsole():
+    PasswordIsDefined(false),
+    NumArchives(0),
+    NumArchiveErrors(0),
+    NumFileErrors(0),
+    NumFileErrorsInCurrentArchive(0),
+    OutStream(NULL),
+    totalSize((UInt64)(Int64)-1),
+    completedSize(0),
+    lastVal(0),
+    ProgressHandler(NULL),
+    ProgressWithFileHandler(NULL)
+  {}
+
+  void Init()
+  {
+    NumArchives = 0;
+    NumArchiveErrors = 0;
+    NumFileErrors = 0;
+    NumFileErrorsInCurrentArchive = 0;
+
+    totalSize = (UInt64)(Int64)-1;
+    completedSize = 0;
+    lastVal = 0;
+    ProgressHandler = NULL;
+    ProgressWithFileHandler = NULL;
+  }
+
+  void UpdateProgress();
+};
+
+#endif