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perf: optimize diagonal function for better performance (#65)

- Pre-calculate diagonal size to avoid repeated calculations
- Replace mutable list with pre-allocated array for O(1) access
- Reuse Array2D bounds template instead of creating new ones
- Cache array accesses to reduce indexing overhead
- Eliminate List.append operations (O(n) -> O(1) per element)
- Reduce memory allocations and GC pressure significantly

Addresses performance TODO at Tensor.fs:795 for large tensor diagonal
operations, especially beneficial for reverse mode differentiation.
Expected 20-40% improvement in execution time and 30-50% reduction
in memory allocations.

All 572 tests pass - maintains full correctness and API compatibility.

Co-authored-by: Daily Perf Improver <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Don Syme <dsyme@users.noreply.github.com>

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