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/Bundles/Nsis7z/api.h

@@ -0,0 +1,83 @@
+/*
+ * apih
+ * 
+ * This file is a part of NSIS.
+ * 
+ * Copyright (C) 1999-2013 Nullsoft and Contributors
+ * 
+ * Licensed under the zlib/libpng license (the "License");
+ * you may not use this file except in compliance with the License.
+ * 
+ * Licence details can be found in the file COPYING.
+ * 
+ * This software is provided 'as-is', without any express or implied
+ * warranty.
+ */
+
+#ifndef _NSIS_EXEHEAD_API_H_
+#define _NSIS_EXEHEAD_API_H_
+
+// Starting with NSIS 2.42, you can check the version of the plugin API in exec_flags->plugin_api_version
+// The format is 0xXXXXYYYY where X is the major version and Y is the minor version (MAKELONG(y,x))
+// When doing version checks, always remember to use >=, ex: if (pX->exec_flags->plugin_api_version >= NSISPIAPIVER_1_0) {}
+
+#define NSISPIAPIVER_1_0 0x00010000
+#define NSISPIAPIVER_CURR NSISPIAPIVER_1_0
+
+// NSIS Plug-In Callback Messages
+enum NSPIM 
+{
+	NSPIM_UNLOAD,    // This is the last message a plugin gets, do final cleanup
+	NSPIM_GUIUNLOAD, // Called after .onGUIEnd
+};
+
+// Prototype for callbacks registered with extra_parameters->RegisterPluginCallback()
+// Return NULL for unknown messages
+// Should always be __cdecl for future expansion possibilities
+typedef UINT_PTR (*NSISPLUGINCALLBACK)(enum NSPIM);
+
+// extra_parameters data structures containing other interesting stuff
+// but the stack, variables and HWND passed on to plug-ins.
+typedef struct
+{
+  int autoclose;
+  int all_user_var;
+  int exec_error;
+  int abort;
+  int exec_reboot; // NSIS_SUPPORT_REBOOT
+  int reboot_called; // NSIS_SUPPORT_REBOOT
+  int XXX_cur_insttype; // depreacted
+  int plugin_api_version; // see NSISPIAPIVER_CURR
+                          // used to be XXX_insttype_changed
+  int silent; // NSIS_CONFIG_SILENT_SUPPORT
+  int instdir_error;
+  int rtl;
+  int errlvl;
+  int alter_reg_view;
+  int status_update;
+} exec_flags_t;
+
+#ifndef NSISCALL
+#  define NSISCALL __stdcall
+#endif
+
+typedef struct {
+  exec_flags_t *exec_flags;
+  int (NSISCALL *ExecuteCodeSegment)(int, HWND);
+  void (NSISCALL *validate_filename)(TCHAR *);
+  int (NSISCALL *RegisterPluginCallback)(HMODULE, NSISPLUGINCALLBACK); // returns 0 on success, 1 if already registered and < 0 on errors
+} extra_parameters;
+
+// Definitions for page showing plug-ins
+// See Ui.c to understand better how they're used
+
+// sent to the outer window to tell it to go to the next inner window
+#define WM_NOTIFY_OUTER_NEXT (WM_USER+0x8)
+
+// custom pages should send this message to let NSIS know they're ready
+#define WM_NOTIFY_CUSTOM_READY (WM_USER+0xd)
+
+// sent as wParam with WM_NOTIFY_OUTER_NEXT when user cancels - heed its warning
+#define NOTIFY_BYE_BYE 'x'
+
+#endif /* _PLUGIN_H_ */