[Enhancement] Expose cluster_mask on T.tma_copy (#2932)

* [Enhancement] Expose cluster_mask on T.tma_copy

TMA multicast already composes with tma_copy's split-phase contract: in
LowerBulkCopy, use_multicast is orthogonal to the flag that suppresses the
wait, so annotations={"cluster_mask": ...} produces an issue-only multicast
today. Only the named parameter was missing.

Promote it to a documented keyword argument, matching the parameter that
copy_cluster already exposes under the same name, and write down the
semantics: the lowest-ranked CTA in the mask issues the multicast, other
in-mask CTAs issue nothing, a CTA outside the mask falls back to a unicast
load, and every CTA runs its own expect_tx against its local barrier.

No lowering change.

* [Testing] Drop the duplicated TMA store arrive/wait

T.copy already lowers shared -> global to tma_store + tma_store_arrive +
tma_store_wait, so the explicit pair after it emitted a second, redundant
arrive/wait.