Keep broadcasts over views of dense arrays sparse

[ViralBShah]

Fixes #508.

Broadcasting a sparse array against a dense Matrix stays sparse, but doing
the same against a view of that matrix silently produced a dense result:

julia> A = sprand(5, 5, 0.3); B = rand(5, 5);

julia> typeof(A .* B)
SparseMatrixCSC{Float64, Int64}

julia> typeof(A .* view(B, :, :))
Matrix{Float64}

The cause is is_supported_sparse_broadcast, which whitelists Array but
not SubArray. A view therefore answered false, and copy(::Broadcasted{PromoteToSparse})
diverted the whole expression to the generic dense path — losing both the
sparse result type and the structural zeros of A.

This recurses through SubArray to its parent, exactly as Transpose/Adjoint
are already handled one line above, so a view is supported whenever the array
it views is. That covers non-strided views (view(B, [1,3,5], :)) as well as
strided ones, and picks up views of sparse arrays at the same time. Arrays
that were unsupported before still divert to the dense fallback.

Tests cover dense views, adjoints of views, sparse views, broadcast!, and
that a zero-preserving op no longer fills in structural zeros.

🤖 Generated with Claude Code

https://claude.ai/code/session_01L8HenXTVrASo1sBZVGhpDQ

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.19%. Comparing base (53690b3) to head (5b4df74).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #772   +/-   ##
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  Coverage   84.19%   84.19%           
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  Files          13       13           
  Lines        9403     9404    +1     
=======================================
+ Hits         7917     7918    +1     
  Misses       1486     1486           

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