Summary

This PR improves assignment performance for dynamic-rank broadcast and permutation expressions, focusing on the stepper-heavy workloads reported in #2784, #2683, and #2859.

It adds two conservative execution paths:

1. Runtime pointer/stride planning for supported 3-D broadcast expressions.
2. Source-ordered permutation kernels for dense 3-D transpose/cast/scaling expressions.

Unsupported types, dimensions, layouts, views, and stride patterns retain the existing assignment path. The PR also adds comparison benchmarks for raw loops, Eigen, Armadillo (when available), dynamic shapes, fixed shapes, broadcasting, and transpose/cast expressions.

Runtime broadcast plan

For non-trivial three-dimensional row-major broadcasts with directly addressable arithmetic leaves:

• Flatten the expression tree once into typed leaf/function plans.
• Cache leaf pointers and aligned per-axis strides.
• Represent broadcast axes as zero strides.
• Distinguish zero and unit innermost strides so the compiler can vectorize the generated loop.
• Retain the original assignment path for unsupported expressions and layouts.

This avoids rebuilding/reseeking recursive steppers for every short inner row.

Permutation kernels

For dense 3-D cast(transpose(...)) / scalar assignments:

• Determine source-contiguous loop order once from runtime strides.
• Use a 32x32 tiled fallback for general dense permutations.
• Use a source-ordered RGB deinterleave path for interleaved 3-channel input and planar output.
• Preserve exact division semantics rather than replacing division with reciprocal multiplication.
• Fall back to the existing assignment machinery when the expression or stride pattern is not eligible.

Benchmarks

benchmark_compare.cpp covers dynamic contiguous fused arithmetic, dynamic N-D broadcasting, fixed-shape fused arithmetic, transpose/cast/scaling, raw loops, Eigen Array/Tensor, and optional Armadillo comparisons.

Measured with -O3 -march=native, xsimd 14, AVX-512, SIMD width 8:

Validation

• Full non-SIMD suite: 78/78 tests passed.
• Full SIMD suite: 78/78 tests passed (20,213 assertions in aggregate xtest).
• Pre-commit hooks pass.
• Added coverage for dense transpose/cast assignment, RGB deinterleave, general tiled permutation, runtime broadcast planning, and unsupported-expression fallback.