build: redirect 26.00 builds to bundle directory

- tools/linux/build_plugin_linux.py: point VERSION_LAYOUT['26.00'].bundle_dir
  to versions/26.00-bundle/ (was versions/26.00/); update module docstring.
- tools/legacy/build_plugin_2600_vs2026.py: point project_dir to
  versions/26.00-bundle/ (was versions/26.00/).
- tools/linux/setup_26_01_bundle_symlinks.py: refactor to use bundle_root /
  vendor_root variables and add C/Asm root-level symlink creation, matching
  the new setup_26_00_bundle_symlinks.py convention.

tools/linux/build_plugin_linux.py

@@ -24,10 +24,14 @@ BUILD_DIR = ROOT / "_linux_build"
 
 SUPPORTED = {"25.01", "26.00", "26.01", "zstd"}
 
-# For 25.01/26.00 the vendor bundle dir already contains our wrapper .cpp
-# files alongside the upstream 7-zip sources.  make is run with -C pointing
-# there so relative paths (../../UI/…) resolve correctly, but the makefile
-# itself comes from OVERLAY and the output objects go to BUILD_DIR.
+# For 25.01 the vendor bundle dir already contains our wrapper .cpp files
+# alongside the upstream 7-zip sources.  make is run with -C pointing there so
+# relative paths (../../UI/…) resolve correctly, but the makefile itself comes
+# from OVERLAY and the output objects go to BUILD_DIR.
+#
+# For 26.00, 26.01: the upstream vendor does NOT ship a Nsis7z bundle, so a
+# *-bundle wrapper directory mirrors the vendor tree via symlinks and provides
+# project-owned sources (NSIS UI, wrapper cpp files, vcxproj).
 #
 # For zstd the 7-zip C++ sources live in the 7-zip-zstd submodule while our
 # NSIS wrapper sits in versions/zstd/.  make is run with -C pointing to the
@@ -40,7 +44,7 @@ VERSION_LAYOUT = {
     },
     "26.00": {
         "vendor_7zip": ROOT / "versions" / "26.00" / "CPP" / "7zip",
-        "bundle_dir":  ROOT / "versions" / "26.00" / "CPP" / "7zip" / "Bundles" / "Nsis7z",
+        "bundle_dir":  ROOT / "versions" / "26.00-bundle" / "CPP" / "7zip" / "Bundles" / "Nsis7z",
     },
     "26.01": {
         "vendor_7zip": ROOT / "versions" / "26.01" / "CPP" / "7zip",