Implement aggregate rendezvous lowering for uniform multicast PipeNets

[brnorris03]

Parent: #619

Problem

Uniform multicast PipeNets currently contribute to the hardware semaphore pressure tracked in #619. The current lowering can allocate rendezvous resources per receiver or per copy site, even when all receivers publish an equivalent destination DFB address/offset.

Proposed lowering

For multicast cases where all receivers publish an equivalent destination DFB address/offset, lower the protocol like tt-metal multicast patterns:

• receivers reserve the destination DFB slot;
• receivers increment one sender-ready counter;
• sender waits for num_dests;
• sender multicasts data to the uniform destination address;
• sender multicasts one receiver-valid/completion signal.

This should make semaphore usage proportional to multicast streams/epochs rather than receiver count or copy-site count.

References

• tt-metal matmul multicast sender waits for num_dests, multicasts data, then multicasts the receiver-valid signal:
  https://github.com/tenstorrent/tt-metal/blob/9938a888cc4efd766d7652c08ab7eeb8fedd9aaf/tt_metal/programming_examples/matmul/matmul_common/kernels/dataflow/reader_bmm_tile_layout_in0_sender_in1_sender.cpp#L127-L161
• tt-metal matmul multicast receivers reserve locally, increment the sender counter, wait for valid, then push the DFB:
  https://github.com/tenstorrent/tt-metal/blob/9938a888cc4efd766d7652c08ab7eeb8fedd9aaf/tt_metal/programming_examples/matmul/matmul_common/kernels/dataflow/reader_bmm_tile_layout_in0_receiver_in1_receiver.cpp#L76-L89
• Conv multicast uses the same aggregate ready/valid structure with a small fixed semaphore set:
  https://github.com/tenstorrent/tt-metal/blob/9938a888cc4efd766d7652c08ab7eeb8fedd9aaf/ttnn/cpp/ttnn/operations/conv/conv2d/device/conv2d_op_sharded_program_factory.cpp#L714-L724
  https://github.com/tenstorrent/tt-metal/blob/9938a888cc4efd766d7652c08ab7eeb8fedd9aaf/ttnn/cpp/ttnn/operations/conv/conv2d/device/kernels/activation_reader_width_sharded.cpp#L207-L271

Non-goals

This does not solve non-uniform destination DFB addresses. Those still need mailbox/table-based address publication.