Remove name shadows, dead aliases, and unused imports in linalg and broadcasting

[ViralBShah]

Cleanup from a structural review of src/linalg.jl and src/higherorderfns.jl. No behaviour change.

Name shadows. These are the same class of bug as #769 (findprev defined locally instead of extending Base), just with internal names:

• src/linalg.jl defined an unqualified _generic_matmatmul!, which creates a SparseArrays-local function, while the same file calls LinearAlgebra._generic_matmatmul! (qualified) for dense destinations. Two different functions with one name, 350 lines apart. The local one is renamed _generic_spmatmatmul! with a comment explaining the relationship.
• Likewise the local _mul! used by the symmetric/Hermitian path (now _symherm_mul!) versus the extension of LinearAlgebra._mul! at the top of the file.
• The pre-1.10 top_set_bit shim shadowed Base.top_set_bit; dropped in favour of calling Base.top_set_bit directly (the package requires Julia 1.11).
• const DenseVecOrMat shadowed the exported Base.DenseVecOrMat with a different meaning, and was unused.

Dead code. DenseTriangular, SpBroadcasted2, and a nine-line commented-out factorize method, all with zero references.

Missing const. SparseVecOrMat (used in 32 method signatures) and SparseOrStructuredMatrix in higherorderfns.jl were plain globals, unlike the five aliases next to them.

Imports. Removed StridedMaybeAdjOrTransMat, SorF, and diagm, none of which are referenced. isbanded is called but never extended, and it is not exported by LinearAlgebra, so it moves from import to using.

Full test suite passes on nightly; Test.detect_ambiguities(SparseArrays) remains empty; whitespace check clean.

🤖 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 84.20%. Comparing base (305ac27) to head (778d464).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #779      +/-   ##
==========================================
+ Coverage   84.19%   84.20%   +0.01%     
==========================================
  Files          13       13              
  Lines        9403     9403              
==========================================
+ Hits         7917     7918       +1     
+ Misses       1486     1485       -1     

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