Implement asynchronous stat() on File class returning a new Stat object with device, inode, mode, linkCount, user, group, specialDevice, size, blockSize, and blockCount fields populated from libuv uv_fs_stat callback. Register foreign method statPath_ in module registry, bump MAX_METHODS_PER_CLASS to 10, add Stat class to io.wren with indexed field accessors, update documentation index with Stat page, and include test coverage for stat on existing file and nonexistent path.
Restructure the documentation hierarchy to prepare for documenting additional built-in modules. Move all core library documentation from `doc/site/core/` to `doc/site/modules/core/`, rename `modules.markdown` to `modularity.markdown`, remove `^category` metadata lines from all affected pages, update cross-reference links to use relative paths under the new structure, and add a TODO placeholder to the Class class page.
