Fix MjSpec zip includes, deep-copy attach assets, and WASM lifetimes

[Ayush7614]

Summary

• MjSpec.from_zip now keeps sibling XML members and passes them as include= after selecting a non-included root XML (previously every .xml entry overwrote the root and includes were dropped).
• Deep-copy attach prefixes mesh/texture file fields on the parent after attach, so in-memory assets resolve; the child stays reusable for later deep-copy attaches.
• WASM: default MjvScene() initializes model = nullptr and returns empty views instead of null-dereferencing model-backed accessors; MjVFS destructor deletes the wrapper after mj_deleteVFS.

Distinct from #3104 / PR #3372 (those cover to_zip file-referenced assets). No existing issue claimed for these paths.

Test plan

• from_zip still loads testdata/MJCF_Root.zip and MJCF_NoRoot.zip
• New include-containing zip round-trips and compiles (test_from_zip_with_includes)
• Deep-copy attach asset failure reproduced before the fix; regression test added (test_deepcopy_attach_renames_parent_asset_files) — needs CI Python bindings build
• CI Python + WASM codegen checks on this PR

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