SDMA is Triton-only, and no collective uses it

Checked on main @ 6462f028.

Where SDMA exists today

iris/mem/triton/ops.py exposes copy_engine_ctx / use_copy_engine on four
entry points:

Where it does not

Gluon has no SDMA surface. iris/mem/gluon/context.py mirrors the Triton RMA
set — load, store, get, put, copy, and nine atomics — but none of them
take a copy-engine argument, and sdma appears nowhere under iris/mem/gluon/.

No collective uses SDMA on either backend. grep -r "sdma\|copy_engine" iris/ccl/ returns nothing. Every kernel under iris/ccl/triton/ and
iris/ccl/gluon/ moves bytes with CU loads and stores.

So SDMA is available to someone hand-writing a Triton kernel against the RMA
primitives, and to nobody else. ctx.ccl.* cannot reach it.

Note the shape this leaves us in: the Gluon backend does have TDM
(iris/ccl/gluon/all_gather_tdm.py, #585, gfx1250), so Gluon got the newer
data-movement engine but not MI300-series copy engines, and the collectives got
neither.

Scope

Two pieces, in order:

1. Gluon copy-engine surface — add copy_engine_ctx / use_copy_engine to
   IrisDeviceCtx.put (and the atomics that have it on the Triton side), matching
   the Triton signatures so kernels can move between backends.
2. An SDMA collective — one collective, one backend, as a proof point. All-gather
   is the natural first: the transfer is a contiguous per-destination copy, which
   is exactly the contiguous_copy=True path put already supports.

Work

• Mirror copy_engine_ctx / use_copy_engine in iris/mem/gluon/context.py
• Add a copy-engine all-gather variant behind all_gather_variant="sdma"
• Decide whether Config grows an engine axis or SDMA stays a variant name
• Correctness vs torch.distributed on 2/4/8 ranks before timing
• Benchmark vs the CU-driven variants through benchmark/ccl/bench_all_gather.py,
  which already has a comm_sms axis — the interesting number is CU count at
  equal bandwidth, not bandwidth at equal CU count

Related

#568 (engines= / copy-engine API), #573 (triggered SDMA), #585 (TDM all-gather).