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/UserInputUtils2.cpp

@@ -0,0 +1,60 @@
+// UserInputUtils.cpp
+
+#include "StdAfx.h"
+
+#include "Common/StdInStream.h"
+#include "Common/StringConvert.h"
+
+#include "UserInputUtils2.h"
+
+static const char kYes = 'Y';
+static const char kNo = 'N';
+static const char kYesAll = 'A';
+static const char kNoAll = 'S';
+static const char kAutoRename = 'U';
+static const char kQuit = 'Q';
+
+static const char *kFirstQuestionMessage = "?\n";
+static const char *kHelpQuestionMessage =
+  "(Y)es / (N)o / (A)lways / (S)kip all / A(u)to rename / (Q)uit? ";
+
+// return true if pressed Quite;
+// in: anAll
+// out: anAll, anYes;
+
+NUserAnswerMode::EEnum ScanUserYesNoAllQuit2(CStdOutStream *outStream)
+{
+  (*outStream) << kFirstQuestionMessage;
+  for(;;)
+  {
+    (*outStream) << kHelpQuestionMessage;
+	AString scannedString;
+	g_StdIn.ScanAStringUntilNewLine(scannedString);
+    scannedString.Trim();
+    if(!scannedString.IsEmpty())
+      switch(::MyCharUpper(scannedString[0]))
+      {
+        case kYes:
+          return NUserAnswerMode::kYes;
+        case kNo:
+          return NUserAnswerMode::kNo;
+        case kYesAll:
+          return NUserAnswerMode::kYesAll;
+        case kNoAll:
+          return NUserAnswerMode::kNoAll;
+        case kAutoRename:
+          return NUserAnswerMode::kAutoRename;
+        case kQuit:
+          return NUserAnswerMode::kQuit;
+      }
+  }
+}
+
+UString GetPassword(CStdOutStream *outStream)
+{
+  (*outStream) << "\nEnter password:";
+  outStream->Flush();
+  AString oemPassword;
+  g_StdIn.ScanAStringUntilNewLine(oemPassword);
+  return MultiByteToUnicodeString(oemPassword, CP_OEMCP);
+}