Phase 5 — trn2 PSUM sizing for larger batched tile generation

[scttfrdmn]

Roadmap phase tracker

This issue tracks trnrand'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 trnrand:
Wider PSUM on trn2 means larger random-tile batches per kernel call.

Done means:

• trn2-specific Philox path with larger partition count.
• Benchmarks show the throughput win on trn2 vs the common-path baseline.
• Runtime detection wires the right kernel per detected hardware.

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.