[TE][CXI]: Split transfers that straddle BufferDesc chunk boundaries

Parent: #3500

## Problem

`CxiTransport::registerLocalMemory` already auto-splits buffers larger than `max_mr_size` into multiple `BufferDesc`s.

`CxiTransport::submitTransferTask` still posts **one slice per request**. A transfer that crosses a chunk seam fails `selectDevice`.

Until this is fixed, Store must keep splitting CXI segments in `GetTransportRegistrationLimit()`.

## Proposed work

Same as EFA / RDMA: cap each posted slice at source and target `BufferDesc` remainder, then bind keys per sub-slice.

## Test plan

Needs CXI / Slingshot hardware. Suggested coverage matching `rdma_large_mr_test.cpp` (destination seam, source seam, past-first-chunk WRITE).

I do not have CXI hardware to implement or verify this.

Start with CxiTransport::registerLocalMemory and CxiTransport::submitTransferTask, then compare the proposed slicing behavior with the EFA/RDMA implementation. Review rdma_large_mr_test.cpp for destination-seam, source-seam, and past-first-chunk WRITE coverage. Done means each posted slice respects both BufferDesc remainders and binds keys per sub-slice, with CXI hardware tests passing.

Written by the indexing model from the issue text.