feat(26.01): add bundle wrapper directory with NSIS sources and vendor symlinks

7-Zip 26.01 (ip7z/7zip) never shipped NSIS plugin sources, so they live in
a separate bundle directory (versions/26.01-bundle/) that mirrors the layout
expected by the shared overlay makefile and by MSBuild.

Bundle contents:
- CPP/7zip/Bundles/Nsis7z/: NSIS plugin wrapper sources (nsis7z.cpp,
  pluginapi.cpp, api.h, resource.rc, StdAfx.h, …) and the Windows build
  project file (Nsis7z_vs2026.vcxproj, adapted from versions/26.00/).
  The vcxproj references the vendor submodule tree via relative paths
  (../../../../../26.01/…), keeping the submodule pristine.
- CPP/7zip/UI/NSIS/: NSIS UI adapter sources (Main.cpp, MainAr.cpp,
  NSISBreak.cpp, ExtractCallbackConsole.cpp, UserInputUtils2.cpp and
  corresponding headers), copied from versions/26.00/.
- CPP/7zip/LzmaDec_gcc.mak: forwarding stub; GNU make resolves relative
  `include` paths against the CWD, so when make runs from Bundles/Nsis7z
  the upstream Arc_gcc.mak's `include ../../LzmaDec_gcc.mak` lands here.
  The stub re-includes the real file via $(VENDOR_7ZIP).
- Symlinks (C, Asm, CPP/7zip/*, CPP/Common, CPP/Windows, …) pointing into
  the versions/26.01 submodule.  These are required because the vendor
  makefile uses CWD-relative source paths (../../../../C/…, ../../Common/…).
  tools/linux/setup_26_01_bundle_symlinks.py recreates them if needed.

versions/26.01-bundle/CPP/7zip/Bundles/Nsis7z/pluginapi.h

@@ -0,0 +1,115 @@
+#ifndef ___NSIS_PLUGIN__H___
+#define ___NSIS_PLUGIN__H___
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "api.h"
+#ifdef _UNICODE
+#ifndef _T
+#define __T(x)   L ## x
+#define _T(x)    __T(x)
+#define _TEXT(x) __T(x)
+#endif
+#else
+#ifndef _T
+#define _T(x)    x
+#define _TEXT(x) x
+#endif
+#endif
+
+#ifndef NSISCALL
+#  define NSISCALL __stdcall
+#endif
+
+#define EXDLL_INIT()           {  \
+        g_stringsize=string_size; \
+        g_stacktop=stacktop;      \
+        g_variables=variables; }
+
+typedef struct _stack_t {
+  struct _stack_t *next;
+  TCHAR text[1]; // this should be the length of string_size
+} stack_t;
+
+enum
+{
+INST_0,         // $0
+INST_1,         // $1
+INST_2,         // $2
+INST_3,         // $3
+INST_4,         // $4
+INST_5,         // $5
+INST_6,         // $6
+INST_7,         // $7
+INST_8,         // $8
+INST_9,         // $9
+INST_R0,        // $R0
+INST_R1,        // $R1
+INST_R2,        // $R2
+INST_R3,        // $R3
+INST_R4,        // $R4
+INST_R5,        // $R5
+INST_R6,        // $R6
+INST_R7,        // $R7
+INST_R8,        // $R8
+INST_R9,        // $R9
+INST_CMDLINE,   // $CMDLINE
+INST_INSTDIR,   // $INSTDIR
+INST_OUTDIR,    // $OUTDIR
+INST_EXEDIR,    // $EXEDIR
+INST_LANG,      // $LANGUAGE
+__INST_LAST
+};
+
+extern unsigned int g_stringsize;
+extern stack_t **g_stacktop;
+extern TCHAR *g_variables;
+
+void NSISCALL pushstring(const TCHAR *str);
+void NSISCALL pushintptr(INT_PTR value);
+#define pushint(v) pushintptr((INT_PTR)(v))
+int NSISCALL popstring(TCHAR *str); // 0 on success, 1 on empty stack
+int NSISCALL popstringn(TCHAR *str, int maxlen); // with length limit, pass 0 for g_stringsize
+INT_PTR NSISCALL popintptr();
+#define popint() ( (int) popintptr() )
+int NSISCALL popint_or(); // with support for or'ing (2|4|8)
+INT_PTR NSISCALL nsishelper_str_to_ptr(const TCHAR *s);
+#define myatoi(s) ( (int) nsishelper_str_to_ptr(s) ) // converts a string to an integer
+unsigned int NSISCALL myatou(const TCHAR *s); // converts a string to an unsigned integer, decimal only
+int NSISCALL myatoi_or(const TCHAR *s); // with support for or'ing (2|4|8)
+TCHAR* NSISCALL getuservariable(const int varnum);
+void NSISCALL setuservariable(const int varnum, const TCHAR *var);
+
+#ifdef _UNICODE
+#define PopStringW(x) popstring(x)
+#define PushStringW(x) pushstring(x)
+#define SetUserVariableW(x,y) setuservariable(x,y)
+
+int  NSISCALL PopStringA(char* ansiStr);
+void NSISCALL PushStringA(const char* ansiStr);
+void NSISCALL GetUserVariableW(const int varnum, wchar_t* wideStr);
+void NSISCALL GetUserVariableA(const int varnum, char* ansiStr);
+void NSISCALL SetUserVariableA(const int varnum, const char* ansiStr);
+
+#else
+// ANSI defs
+
+#define PopStringA(x) popstring(x)
+#define PushStringA(x) pushstring(x)
+#define SetUserVariableA(x,y) setuservariable(x,y)
+
+int  NSISCALL PopStringW(wchar_t* wideStr);
+void NSISCALL PushStringW(wchar_t* wideStr);
+void NSISCALL GetUserVariableW(const int varnum, wchar_t* wideStr);
+void NSISCALL GetUserVariableA(const int varnum, char* ansiStr);
+void NSISCALL SetUserVariableW(const int varnum, const wchar_t* wideStr);
+
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif//!___NSIS_PLUGIN__H___