Add <stem>/model.xml search path to mjz_decoder.

PiperOrigin-RevId: 952128127
Change-Id: I8a514297d84026a6028b2416e43df9b5a2a92bbb

doc/changelog.rst

@@ -152,8 +152,8 @@ Compiler
 - Fixed loading of :ref:`.mjz <MJZArchives>` archives in :ref:`simulate<saSimulate>`: the archive was unmounted
   before model compilation, so assets failed to load. Failures in the :ref:`mjz <MJZArchives>` decoder now emit a
   warning with the underlying error instead of the generic "could not decode content" message.
-- The :ref:`mjz <MJZArchives>` decoder now searches for ``model.xml`` at the root of the archive as a fallback if the
-  archive-named XML is not found.
+- The :ref:`mjz <MJZArchives>` decoder now searches for ``model.xml`` and ``<stem>/model.xml`` as a fallback if
+  ``<stem>.xml`` and ``<stem>/<stem>.xml`` are not found.
 - Added support for resource writing via :ref:`mju_writeResource` and the ``write`` callback in
   :ref:`mjpResourceProvider`.
 

src/xml/mjz/mjz_decoder.cc

@@ -89,15 +89,17 @@ class ZipArchiveProvider : public mjpResourceProvider {
 
     // Look for the root XML model in the archive. We try the following
     // locations:
-    // 1. [archive_name].xml at the root of the archive.
-    // 2. [archive_name]/[archive_name].xml inside a subdirectory.
-    // 3. model.xml at the root of the archive (common zipped MJCF pattern).
+    // 1. [archive_name].xml
+    // 2. [archive_name]/[archive_name].xml
+    // 3. model.xml
+    // 4. [archive_name]/model.xml
     const std::filesystem::path path(name_);
     const std::string stem = path.stem().string();
     std::vector<std::string> candidates = {
         (path / stem).generic_string() + ".xml",
         (path / stem / stem).generic_string() + ".xml",
         (path / "model").generic_string() + ".xml",
+        (path / stem / "model").generic_string() + ".xml",
     };
 
     bool found = false;