Bump sparsediff_linalg to v0.6.0

[dance858]

Summary

This PR pins sparsediff_linalg to v0.6.0, up from v0.5.1. No engine source change: the engine calls the matmul dispatchers only (BTA_matrices_*, ATDA_matrices_*, BA_kron_*, sum_n_matrices_*), and none of the changed or removed symbols appear outside notes/.

It takes in library PRs #34–#38:

library PR	change
#34	BA_dense_kron_matrices_* / BA_sparse_kron_matrices_* made private (the engine uses BA_kron_matrices_* since #167)
#35, #37	compact-CSC alloc and gather fill for dense block × sparse (B @ S), then 4 rows of B per visited entry in BA_pd_csr_fill_values
#36	row-wise Gustavson core for sparse B^T A and A^T A
#38	sparse B A and kron(I_p, A) J on the Gustavson core; the dot-form kernels dropped

Public API changes in the library, none of which the engine calls directly:

• BTA_alloc now takes (CSR A, CSC B);
• BTDA_fill_values gained a double *acc argument;
• the private kernels csr_csc_matmul_* and block_left_multiply_fill_* were removed.

Testing

• Debug and Release: all 307 engine tests and all 214 library tests pass.
• The only compiler warnings are 4 -Wcast-qual warnings, all inside the library.
• git grep finds no engine references to the changed or removed symbols outside notes/.

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