Add <kindred> elements to ztools (priority=50) and silo (priority=60)
package.xml files declaring min_create_version, load_priority,
pure_python, and context metadata.
Fix addon_loader.py topological sort to use level-by-level processing
with (priority, name) sorting within each level, replacing
static_order() which did not guarantee deterministic ordering.
Add docs/src/development/package-xml-schema.md documenting the full
field reference, schema example, and backward compatibility notes.
Closes#252
