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/linux/overlay/makefile.gcc

@@ -73,9 +73,15 @@ CURRENT_OBJS = \
   $O/pluginapi.o \
   $O/nsis7z.o \
 
+# Optional extra system objects – pass from the command line when the vendor
+# requires additional translation units that are normally excluded for mingw
+# (e.g. MyWindows.o for the 7-zip-zstd variant).
+EXTRA_SYS_OBJS ?=
+
 OBJS = \
   $(ARC_OBJS) \
   $(SYS_OBJS) \
+  $(EXTRA_SYS_OBJS) \
   $(NSIS_UI_OBJS) \
   $(CURRENT_OBJS) \