Record: Order-20 Dirichlet Posterior + Phrase Cache — 0.11545 BPB (3-seed)

[dentity007]

Order-20 Dirichlet Posterior + Per-Order OBCL + Phrase Cache

val_bpb: 0.11545 (3-seed mean, std 0.0000010) | ~15.1 MB | 8xH100 SXM

Extends n-gram backoff from order 15 (PR #948) to order 20. Improvement validated via 6-test ablation on 1xH100.

3-seed validation (8xH100, Montréal CA, 747 TFLOPS)

Seed	Val BPB	Eval Time
1337	0.11544435	459s
42	0.11546433	435s
2025	0.11544736	438s
Mean	0.11545 (std 0.0000010)

What changed from PR #948

• NGRAM_ORDER=20 (was 15)
• 5 additional per-order concentrations (all 1.86)
• Everything else identical

Compliance

• Training: 560s on 8xH100 (within 600s)
• Eval: 459s worst case (within 600s)
• Artifact: ~15.1 MB (within 16,000,000)
• All caches strictly backward-looking (causal)
• Score-first evaluation

Credits

Same lineage as PR #948. Built on @Robby955 (PR #900), @signalrush (PR #414), @himanshudongre (PR #846), @deanbrr (PR #659), @newjordan (PR #674), @pentxayc (PR #803).