Update README to reflect template-based multi-scalar type support

[Copilot]

PR #4 converted the library to C++ templates, enabling all four scalar types (float, double, complex<float>, complex<double>) simultaneously without recompilation. The README still documented the old CSR4MPI_VALUE_TYPE compile-time switch approach.

Changes

• Project description: Emphasize template-based design; remove "冷切换" (cold-switch) terminology
• Features section: Add template classes, type aliases (cCSRMatrixF/D/CF/CD), and type traits (is_complex_v, is_supported_scalar_v, real_type_t)
• Build section: Remove CSR4MPI_VALUE_TYPE parameter; note that all types are available in single build
• New "标量类型使用" section: Type alias table, usage examples, type traits documentation
• Directory overview: Update to reflect templated headers and new test_all_scalar_types.cpp

Usage Example

using namespace csr4mpi;

// All types available simultaneously
cCSRMatrixD matDouble(0, n, n, rowPtr, colInd, valuesDouble);
cCSRMatrixCF matComplexFloat(0, n, n, rowPtr, colInd, valuesComplexFloat);

std::vector<double> xD(n, 1.0), yD;
SpMV(matDouble, xD, yD);

Original prompt

根据上一次commit更新的内容，更新 readme，稍微详细一点

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