Add O(nnz) issymmetric, count, ==, reverse, circshift, permutedims and sort for views and lazy adjoints of sparse matrices

[ViralBShah]

A view or a lazy adjoint/transpose of a sparse matrix falls to the generic elementwise method for a number of functions that the matrix itself handles in O(nnz). Found in a soundness review of the whole package; this is the follow-up batch for elementwise fallbacks. These are new methods, not bug fixes.

n = 3000, about 15,000 stored entries, one timed call after warm-up, ms / bytes. The last column is the branch at n = 200,000 with about 10⁶ stored entries, which main cannot run in reasonable time.

Call	main	branch	branch, 10⁶ entries
issymmetric(A')	41.3 / 0	0.03 / 24640	3.3 ms
issymmetric(view(A, :, vec))	46.2 / 0	0.06 / 311552	13.4 ms
count(iszero, A')	44.9 / 0	0.00 / 0	0.14 ms
V == V', V = view(A, :, 1:n)	105.0 / 0	0.36 / 573824	51.5 ms
view(A, :, 1:n) + I	42.3 / 778496	0.10 / 659008	8.9 ms
reverse(A')	45.2 / 778496	0.05 / 286912	7.7 ms
reverse(view(A, :, vec); dims=1)	64.8 / 778496	0.08 / 286912	5.3 ms
rot180(A')	126.7 / 712960	0.06 / 286912	13.1 ms
rotl90(view(A, :, 1:n))	123.2 / 778496	0.12 / 573824	12.9 ms
circshift(A', (1, 1))	124.4 / 778496	0.11 / 573824	11.9 ms
permutedims(view(A, :, vec))	112.9 / 778736	0.08 / 573824	6.5 ms
copy(reshape(A', n, n))	136.1 / 778688	0.08 / 574016	6.5 ms
sort(A'; dims=1)	about 19 s at n = 6000	0.24 / 286944	17.0 ms

count on an adjoint or transpose forwards to the parent with the wrapper composed into the predicate, and allocates nothing. issymmetric/ishermitian on an adjoint or transpose ask the parent; on a view they use the view where getcolptr works on it and its sparse copy otherwise. Everything else (==, isequal, reverse, rot180, rotl90, rotr90, circshift, permutedims, sort, copy of a 2-d reshape, and ± I on a view) takes the O(nnz) sparse copy of the argument and hands it to the existing SparseMatrixCSC kernel; the new allocations in the table are that copy. No dispatch alias is added: the methods are on SparseMatrixCSCSubArray and AdjOrTrans{<:Any,<:AbstractSparseMatrixCSC}, so they cover any 2-d view, row subsets included.

Touches dispatch throughout; each new method has a test that it is the one dispatched to. Not for backport.

Behaviour to know:

• sort(X; dims) on a view or adjoint returns a SparseMatrixCSC, as sort(A) does; the generic method returned a ReshapedArray.
• Results keep stored zeros, as the SparseMatrixCSC kernels do; the generic path dropped them. Values are equal.
• isequal(V, V') for a complex view now agrees with isequal(A, A') on a plain sparse matrix, which treats the conjugate of an unstored zero as zero; dense gives false there because of -0.0im. == is unaffected.

Left out: an adjoint against an adjoint when one parent is a view (V' == A'), copy(V') for a view V, and copy(reshape(x, len)) to one dimension are still elementwise. issymmetric on view(A, :, 1:n), count on views, A' + I and A == A' were already O(nnz).

A ± I on a view checks squareness first, as on main; #1019 makes the plain-matrix method do the same.

Written by Claude Code.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 95.83333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.83%. Comparing base (5f40621) to head (f553257).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/sparsematrix.jl	95.83%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1024      +/-   ##
==========================================
- Coverage   95.63%   94.83%   -0.80%     
==========================================
  Files          16       16              
  Lines        9407     9790     +383     
==========================================
+ Hits         8996     9284     +288     
- Misses        411      506      +95     

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