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/setup_bundle_symlinks.py

@@ -8,6 +8,7 @@ Example:
     python3 setup_bundle_symlinks.py 25.01
     python3 setup_bundle_symlinks.py 26.00
     python3 setup_bundle_symlinks.py 26.01
+    python3 setup_bundle_symlinks.py zstd
 """
 import os
 import sys
@@ -19,9 +20,14 @@ if len(sys.argv) != 2:
 
 version = sys.argv[1]
 
+# For versions where the vendor directory name differs from the bundle version key.
+VENDOR_DIR = {
+    "zstd": "7-zip-zstd",
+}
+
 ROOT = Path(__file__).resolve().parents[2]
 bundle_root = ROOT / f"versions/{version}-bundle"
-vendor_root = ROOT / f"versions/{version}"
+vendor_root = ROOT / "versions" / VENDOR_DIR.get(version, version)
 
 if not vendor_root.exists():
     print(f"Error: vendor directory not found: {vendor_root}", file=sys.stderr)