hexagon: add neg, exp, sigmoid, softplus ops, cont, repeat ops (llama/20701)

hexagon: add neg, exp, sigmoid, softplus ops, cont, repeat ops (llama/20701)

Add element-wise unary ops needed by Qwen 3.5's DeltaNet linear
attention layers. These ops follow the existing unary-ops pattern
with VTCM DMA double-buffering.

- neg: negate via scale by -1.0
- exp: uses existing hvx_exp_f32 HVX intrinsics
- sigmoid: uses existing hvx_sigmoid_f32_aa HVX intrinsics
- softplus: log(1 + exp(x)) scalar fallback
- CONT reuses the existing CPY infrastructure since making a tensor
  contiguous is equivalent to a same-type copy.
- REPEAT implements tiled memory copy with multi-threaded execution via
  the worker pool, supporting f32 and f16 types. The kernel parallelizes
  across output rows and uses memcpy for each tile.

Co-authored-by: Max Krasnyansky <redacted>