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"""Packed-vector emit functions for elementwise ops.

Each module here exposes one or more ``VecImpl`` instances that an ``OpSpec``
(in ``ops/``) attaches to its ``vec_impls`` list. ``reg.py``/``smem.py`` then
pick the widest matching one at dispatch time, mirroring how copy picks
``copy_{Nb}`` from a menu.

VecImpl emit contract:
  emit(dst_buf, dst_lane_indices, src_args, extras) -> Expr

  * dst_buf: Buffer
  * dst_lane_indices: list[list[Expr]] of length ``vec_len``; each entry is the
                     multi-dim indices for one lane (precomputed by schedule).
  * src_args[i]: one of
      - Expr (scalar src — broadcast across all lanes)
      - tuple (Buffer, list[list[Expr]] of length ``vec_len``) — buffer src
                                                                with per-lane indices
  * extras: dict (rounding_mode, etc.)

  Returns the PTX/CUDA call result; the schedule wraps in ``T.evaluate`` at
  the call site. All Python-side shape branching (scalar vs buffer src) happens
  in this emit function -- collapses the old 4x2 schema.py factory explosion.
"""
