feat: add einsum() with deterministic accumulation and generic contraction kernels - #34

This PR adds NDArray::einsum() — Einstein summation notation with deterministic accumulation order and a generic contraction engine backed by type-generic kernels.

Motivation and Context

matmul() and dot() delegate to BLAS, which tiles matrix operations for cache efficiency but produces non-deterministic accumulation order. For pipelines involving clamp() → log() or similar threshold-sensitive operations, ~10⁻⁷ differences from BLAS tiling can cascade into wrong results. einsum() fills this gap by offering the same operations with fixed canonical loop order, identical output on every run, every platform.

What's Changed

• einsum(string $subscripts, ?NDArray $other) — two-operand and single-operand Einstein summation
• Subscript parser supporting 12+ patterns: matrix multiply, dot, outer, element-wise, trace, diagonal, transpose, sum over axis, sum all, matrix-vector, vector-matrix, matrix×transposed
• Fallback generic contraction engine for unoptimized subscript patterns
• Global function alias einsum() in PhpMlKit\NDArray\Linalg namespace
• Updated SPEC.md (section 9.5), API docs, and global functions docs

Breaking Changes

None.