Dynamic dimension support for Tensor and Index classes #5
Description
Extend tensors/indexes so that selected dimensions expose runtime size/stride alongside current compile-time (constexpr) descriptors.
Descriptor objects: let DimInfo remain zero-cost static objects while introducing RuntimeDimInfo that stores Dim values; keep a uniform API so expressions stay constexpr whenever all contributors are static.
Impacted areas: core templates (Tensor, Index, Cursor), dim traits utilities, codegen specializations, and unit tests for slicing/indexing; performance-sensitive paths must preserve the static fast path.
Benefits: broader applicability to runtime-shaped data and mixed static/dynamic layouts without rewriting higher-level algorithms.
Risks/effort: substantial refactor touching fundamental types, potential ABI/API adjustments, and need for extensive regression tests; not currently blocking ongoing work.