Phase 5 — trn2 DMA bandwidth exploitation for SpMM

[scttfrdmn]

Roadmap phase tracker

This issue tracks trnsparse's work on Phase 5 of the trnsci roadmap.

See the suite-level roadmap
for the full phase matrix and cross-project dependencies. A reader-friendly
version lives at trnsci.dev/roadmap/.

What this phase means for trnsparse:
SpMM is DMA-bound on uneven-nnz matrices. trn2's DMA engine has
more bandwidth and deeper queues.

Done means:

• trn2-specific SpMM scheduler that saturates the DMA engine while the Tensor Engine runs the dense tile.
• Runtime detection.
• Benchmarks show the DMA-bound ceiling on both generations with the trn2 path above.

Coordination

• Label: phase-5-generation — matches the same label in every sub-project.
• Cross-project dependencies, if any, are called out in the "Done means"
  list above. Link any PRs / child issues here for tracking.

Close this issue when all "Done means" items are satisfied.