feat: run-time index tensors and independent-axis tiling for reductions and data movement

[asteinh]

• int32 index tensors may come from a model input or an ArgMax/ArgMin output and feed only index operands; data stays float32 or int8. ONNX int64 index inputs narrow to int32 at the boundary.
• Gather-family and DynamicUpdateSlice metadata support run-time indices next to the constant form.
• The partitioner tiles reductions, ArgMax/ArgMin and index-based data movement along an axis they leave independent, in height or matrix-row bands. Run-time-indexed operators stay untiled.
• Contract cases: all four index operators with model-input indices, and ArgMax feeding Gather, in float32 and int8.

Needs the runtime branch of the same name.