Return OffDiagonal when slicing Diagonal of LayoutVector (require FillArrays v1.18)

[dlfivefifty]

FillArrays v1.18 adds OffDiagonal and returns it when slicing a Diagonal of an AbstractFill by unit ranges. This PR does the same for a Diagonal whose diagonal is a LayoutVector or a SubArray{<:Any,1,<:LayoutVecOrMat}: before, these slices fell back to Base and returned a dense Matrix.

julia> D = Diagonal(MyVector(randn(5)));

julia> D[2:4, 1:5] isa FillArrays.OffDiagonal
true

Changes:

• Bump the FillArrays compat to 1.18 and the version to 1.15.
• Add getindex(::Diagonal{<:Any,<:Union{LayoutVector,SubArray{<:Any,1,<:LayoutVecOrMat}}}, ::Union{AbstractUnitRange,Colon}, ::Union{AbstractUnitRange,Colon}), which calls FillArrays' internal _onebased_index and _singleband_getindex. Offset and infinite index shapes still go to the generic fallback, as they do in FillArrays.
• Add tests for several range and Colon combinations, empty ranges, out-of-bounds slices, and diagonals backed by views.

Note: the new method relies on FillArrays internals, so a later FillArrays release that renames them would break it.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.53%. Comparing base (0d4c00b) to head (036876d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #285      +/-   ##
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+ Coverage   90.33%   90.53%   +0.19%     
==========================================
  Files          12       12              
  Lines        1987     2028      +41     
==========================================
+ Hits         1795     1836      +41     
  Misses        192      192              

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