NanoVDB: default Mask's copy constructor

[sifakis]

Summary

Mask's only data member is uint64_t mWords[WORD_COUNT], and the hand-written copy
constructor's loop copies exactly that range — so it duplicates the implicit memberwise copy.
Defaulting it leaves the semantics unchanged and makes Mask trivially copyable. sizeof and
standard-layout are unaffected.

Note:

• The copy assignment operator immediately below is already = default; this just finishes
  the pair.
• nanovdb::cuda::Buffer<T, R> requires std::is_trivially_copyable<T>, so Mask currently
  cannot be a buffer element type. With this change, mask sidecars can be held in a typed
  Buffer<Mask<N>> rather than a Buffer<std::byte> plus a cast.

Verification

• is_trivially_copyable becomes true for Mask<3>/Mask<4>/Mask<5>; sizeof (64/512/4096)
  and is_standard_layout are unchanged.
• Copy-construction and assignment still work in device code (the defaulted special member is
  implicitly __host__ __device__, as the already-defaulted operator= beside it relies on).

[harrism]

This is a nice simplification, and it enables replacing some Buffer<std::byte> + cast with Buffer<Mask>. After #2301 merges, I will do a cleanup PR to make those changes.

[kmuseth]

looks good