Fix Enzyme sparse matrix sparsity pattern corruption (issue #835)

[ChrisRackauckas-Claude]

Summary

• Fixes the issue where Enzyme AD with sparse matrices corrupts the primal matrix's sparsity pattern (rowval, colptr)
• Adds sparse-safe helper functions that operate directly on nzval arrays instead of using broadcast operations that can change sparsity
• For dense matrices, falls back to standard broadcast operations

Root Cause

Enzyme.make_zero creates shadow sparse matrices that share the structural arrays (rowval, colptr) with the primal matrix. When the reverse rule executes broadcast operations like dA .-= z * transpose(y), these can change the sparsity pattern of the shadow matrix, and because the structural arrays are shared, this inadvertently corrupts the primal matrix's structure.

Evidence from investigation:

Primal rowval === shadow rowval? true
Primal colptr === shadow colptr? true

Solution

Add sparse-safe helper functions that dispatch on AbstractSparseMatrix:

1. _safe_add!(dst, src): For sparse matrices, adds values via dst.nzval .+= src.nzval
2. _safe_zero!(A): For sparse matrices, zeros via fill!(A.nzval, 0)
3. _sparse_outer_sub!(dA, z, y): For sparse matrices, only accumulates gradients into existing non-zero positions using a direct loop over the CSC structure

These preserve the sparsity pattern by operating only on the nzval array.

Test plan

• Verify sparsity pattern is preserved after AD (rowval, colptr, nnz unchanged)
• Verify matrix can still be displayed without AssertionError: _goodbuffers(S) crash
• Verify dense matrix Enzyme tests still pass (note: existing tests have pre-existing failures unrelated to this change)

Fixes #835

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