Phase 5 — trn2 larger-SBUF butterfly path

[scttfrdmn]

Roadmap phase tracker

This issue tracks trnfft'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 trnfft:
trn2 (NeuronCore v3) has a larger SBUF and wider PSUM than trn1.
That lets butterfly stages operate on larger tiles with fewer
SBUF loads.

Done means:

• Runtime detection of NeuronCore generation in the dispatch layer.
• trn2 butterfly kernel: tile sizes matched to the larger SBUF, fewer inter-stage round-trips.
• Benchmarks split out trn1 vs trn2 numbers with a shared common path as the baseline.
• Documented in docs/architecture.md with a forward-planning section for future NeuronCore generations.

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.