build(linux): introduce overlay makefile infrastructure

Replaces the per-version in-tree build files with a shared overlay that
keeps all project-owned build infrastructure outside the vendor source trees,
making it safe to use vendor directories as pristine git submodules.

Changes:
- Add tools/linux/overlay/makefile.gcc: parameterised makefile that drives
  MinGW cross-compilation; VENDOR_7ZIP, DEF_FILE and NSIS_DIR are all
  overridable from the command line so no files are written into the vendor
  tree at build time.
- Add tools/linux/overlay/include/: case-sensitivity shim headers
  (Windows.h, CommCtrl.h, MAPI.h, NTSecAPI.h, Psapi.h, ShlObj.h) that
  redirect to the lowercase MinGW system headers; also adds
  z7_idecl_noexcept_strip.h which resolves the noexcept-specifier mismatch
  in 7-zip 25.01 NSIS UI code.
- Move Nsis7z.def from versions/26.00/…/Bundles/Nsis7z/ to overlay/ (git
  rename; content unchanged).
- Delete versions/26.00/…/Bundles/Nsis7z/makefile.gcc (superseded by the
  overlay).
- Rewrite tools/linux/build_plugin_linux.py to drive the overlay makefile,
  support multiple 7-zip versions (25.01, 26.00, 26.01, zstd) with a
  VERSION_LAYOUT table, and add NSIS_DIR auto-detection for bundle-based
  builds.
- Add _linux_build/ to .gitignore.

tools/linux/overlay/include/ShlObj.h

@@ -0,0 +1,3 @@
+/* Linux/MinGW cross-build shim: ShlObj.h → shlobj.h */
+#pragma GCC system_header
+#include <shlobj.h>