Specialize Diagonal products and dot for adjoint/transpose of sparse matrices

[ViralBShah]

Fixes #619 and #627. Both are cases where a lazy Adjoint/Transpose of a SparseMatrixCSC fell through to a generic method that ignores sparsity.

#619: A' * D and D * A' with D::Diagonal

Materialize the adjoint (O(nnz)) and reuse the existing CSC-times-Diagonal kernels, mirroring how A' * B is already handled for sparse B. The result stays a SparseMatrixCSC.

On released Julia 1.11 the generic fallback is ~300x slower than A * D (N=1000, density 0.1):

Call	Julia 1.11 stdlib	this PR
A * D	112 µs	143 µs
A' * D	34.6 ms	225 µs
D * A'	35.1 ms	264 µs

On current nightly the generic path already materializes the adjoint, so the gain there is only ~10%; the explicit methods make the fast path independent of LinearAlgebra internals and are worth backporting.

#627: dot(A', B) for sparse A, B

The existing wrapper method walks the stored entries of B and does a binary search into A' for each one. Replace it with a merge: walk the columns of B in order and keep one cursor per column of parent(A). Since the row index being sought in each column of the parent is nondecreasing, the cursors only move forward, giving O(nnz(A) + nnz(B) + n) time with O(n) extra memory and no O(nnz) temporary. dot(B, A') reaches the same kernel through the existing conj(dot(A', B)).

Call (N=1000, density 0.1)	Julia 1.11 stdlib	main (nightly)	this PR
dot(copy(A'), A)	550 µs	606 µs	586 µs
dot(A', A)	28.4 ms	2.0 ms	975 µs

Tests cover both wrappers, real and complex eltypes, mixed eltypes, stored zeros, empty columns, non-square shapes, dimension errors, and timing guards. Full test suite passes on nightly and Test.detect_ambiguities(SparseArrays) remains empty.

🤖 Generated with Claude Code

https://claude.ai/code/session_01LgBHUw9Hp7YW5ub29B4R5y

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.23%. Comparing base (53690b3) to head (e7f7f00).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #770      +/-   ##
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+ Coverage   84.19%   84.23%   +0.04%     
==========================================
  Files          13       13              
  Lines        9403     9428      +25     
==========================================
+ Hits         7917     7942      +25     
  Misses       1486     1486              

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