[liz-badada]

rs[]/ag[] were single buffers tagged by epoch. A rank preempted between pushing its T1 messages and reading its peers' does not hold the peers up: they run the next call, overwrite its pending messages, and it spins forever on the old tag.

Split both buffers into two epoch-parity halves (RS_PAR/AG_PAR in the kernel, RS_BYTES/AG_BYTES x2 on the host, +11 MiB per rank). A rank is never two calls ahead, so two halves suffice. Numerics unchanged; sources.json re-pinned.

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