Add mapreduce_layout hook for reductions on LayoutArrays

[dlfivefifty]

Adds a layout hook for reductions, alongside the existing _norm hook:

mapreduce_layout(::Any, f, op, A, dims; kw...) = invoke(mapreduce, Tuple{Any,Any,AbstractArray}, f, op, A; dims, kw...)
Base.mapreduce(f, op, A::LayoutArray; dims=:, kw...) = mapreduce_layout(MemoryLayout(A), f, op, A, dims; kw...)
Base.mapreduce(f, op, A::SubArray{<:Any,N,<:LayoutArray}; dims=:, kw...) where N = mapreduce_layout(MemoryLayout(A), f, op, A, dims; kw...)

sum, prod, maximum, minimum, extrema and reduce all end in mapreduce, so one overload per layout covers them all. The default falls back to Base, so behaviour doesn't change unless a layout opts in. dims and init are passed through.

Motivation: padded arrays in LazyArrays. For example, sum(zeros(∞,2)) and sum(zeros(2,∞)) currently loop forever, because Base iterates over all the zeros. A LazyArrays PR will overload mapreduce_layout for AbstractPaddedLayout so that it reduces over the padded data only.

any, all and count don't go through mapreduce in Base, so they aren't affected.

Testing:

• Added tests checking that sum, maximum, extrema, prod and mapreduce (with dims and init) match Base for LayoutArrays and their views.
• A custom layout checks that the hook is actually dispatched to.
• The full test suite passes on Julia 1.13 and 1.10, including Aqua.
• The LazyArrays test suite passes with this branch.

Bumps the version to v1.13.0 since this adds a new overloadable function.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.24%. Comparing base (1e4fcc1) to head (15ed221).
⚠️ Report is 12 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #281       +/-   ##
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+ Coverage    0.00%   90.24%   +90.24%     
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  Files          11       12        +1     
  Lines        1858     1969      +111     
===========================================
+ Hits            0     1777     +1777     
+ Misses       1858      192     -1666     

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