Fix findnext/findprev for sparse arrays to extend Base, making hash fast

[ViralBShah]

Fixes #570

Follow-up to #766 and #768, addressing the last relative of isequal that still visited every element: hash.

Root cause. The sparse findnext/findprev methods in src/abstractsparse.jl have been dead code since the JuliaLang/julia import cleanup (JuliaLang/julia#42894) dropped findnext, findprev from the import Base: list. They defined a local SparseArrays.findprev instead of extending Base.findprev, so every call fell through to the generic elementwise method. The existing tests compared against dense results and so never noticed.

Why this matters for hash. Base.hash on large arrays (_hash_fib) skips runs of equal values with findprev(!isequal(elt), A, i). On a sparse array that call is where all the time goes. With the import restored it jumps straight to the previous stored entry.

Call	Before	After
hash of a length 10^9 sparse vector, one stored entry	~1 s	2 µs
hash of a 10^4 x 10^4 sparse matrix, one stored entry	81 ms	4 µs

Hash values are unchanged: this PR does not touch hash itself, so a sparse array still hashes identically to its dense counterpart, and explicitly stored zeros do not affect the value. Verified with randomized checks including NaN and signed zeros, and now covered by tests.

Changes:

• Add findnext, findprev back to the import Base: list.
• Test that the sparse methods actually extend Base, and that findnext/findprev with !isequal(x) predicates agree with dense for NaN and signed zeros, including a timing guard on a 10^9 vector.
• Test that hash of sparse vectors and matrices matches dense (with and without a seed, with stored zeros, NaN and -0.0), plus a timing guard.

Based on #768 because both touch the same test files; it will rebase cleanly onto main once that merges. Full test suite passes on nightly and Test.detect_ambiguities(SparseArrays) remains empty.

🤖 Generated with Claude Code

https://claude.ai/code/session_01LgBHUw9Hp7YW5ub29B4R5y

[ViralBShah]

This should close #570.

@fredrikekre Can I ask you for a review?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.58%. Comparing base (53690b3) to head (2ae3698).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #769      +/-   ##
==========================================
+ Coverage   84.19%   84.58%   +0.39%     
==========================================
  Files          13       13              
  Lines        9403     9401       -2     
==========================================
+ Hits         7917     7952      +35     
+ Misses       1486     1449      -37     

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