Fix fill! to return array instead of nothing

[ChrisRackauckas-Claude]

Summary

• Fixed fill! method to return the modified array instead of nothing, aligning with Julia's standard interface

Testing Done

Tested interface compliance with:

• BigFloat: All operations (construction, QR factorization, linear solve, similar, copy) work correctly with BigFloat element types
• JLArrays: Package correctly reports fast_scalar_indexing status; algorithms appropriately require CPU arrays
• ArrayInterface.jl: All compatibility checks pass including fast_scalar_indexing, can_setindex, and qr_instance

All existing tests pass.

Notes

The package has excellent interface compliance overall:

• Proper use of eltype and similar for type genericity
• Element type preservation through all operations
• Correct ArrayInterface.fast_scalar_indexing implementation

cc @ChrisRackauckas

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