Add cuDSS-style block CSR interface for solver creation

[robtaylor]

Summary

Adds a cuDSS-style CSR interface for creating solvers from block CSR matrices:

New Files

• CsrTypes.h: Enums for MatrixType, MatrixView, IndexBase, IndexType
• CsrSolver.h/cpp: BlockCsrDescriptor struct and createSolverFromBlockCsr() function
• CsrSolverTest.cpp: Comprehensive unit tests

API

struct BlockCsrDescriptor {
    int64_t numBlocks;
    int64_t numBlockNonzeros;
    const void* rowStart;      // INT32 or INT64
    const void* colIndices;    // INT32 or INT64
    const int64_t* blockSizes;
    IndexType indexType;       // INDEX_INT32, INDEX_INT64
    MatrixType mtype;          // MTYPE_SPD (only SPD supported)
    MatrixView mview;          // MVIEW_LOWER, MVIEW_UPPER, MVIEW_FULL
    IndexBase indexBase;       // BASE_ZERO, BASE_ONE
};

SolverPtr createSolverFromBlockCsr(const Settings& settings, 
                                    const BlockCsrDescriptor& desc, ...);

Features

• Supports both int32 and int64 indices
• Supports zero-based and one-based indexing
• Lower/upper triangular views (auto-transposed as needed)
• loadFromCsr() / extractToCsr() for value mapping with permutation handling

Tests

8 unit tests covering:

• Structure conversion
• Index type handling (INT32/INT64)
• Base handling (zero/one)
• Invalid matrix type rejection
• Full factor+solve workflow

Dependencies

None - this PR is independent and can be merged at any time.

Test Plan

• All 8 CSR solver tests pass
• Existing tests unaffected

🤖 Generated with Claude Code

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