…confirms §25 corpus-diversity hypothesis — v2.77 → v2.78

P1 corpus pipeline complete end-to-end. P2 MODEL-2 retrain on 565.6M-token
codeparrot Python+permissive corpus (7.6× the 4× CSN-Python baseline)
pushes val_loss from the 9.7507 plateau to 9.3837 — a 0.367-nat (4.7%)
improvement with the SAME training configuration.

§25 had concluded (after 80K-step LR-budget falsification on 4× CSN-Python):
  "There is no LR/step configuration that beats val_loss=9.75 on
   CSN-Python — only Stack v2 will move the needle."

§33 confirms this empirically. The corpus-diversity binding criterion of
§26.9 is satisfied.

## Pipeline (all stack-canonical, no muda)

| Phase | Outcome |
|-------|---------|
| P1.0 contract authored (PROPOSED → ACTIVE) | #1080 → #1089 |
| P1.1 apr pull dataset extension | #1089 MERGED |
| P1.4 codeparrot pull | 80 shards / 27 GB |
| P1.5a parquet → JSONL filter | 405,904 rows / 3.17 GB |
| P1.5b BPE encode-corpus | 57 shards / 565.6M tokens / 10h |
| P2 MODEL-2 retrain on RTX 4090 | EARLY_STOP at 51 ep / 47 min |

Total wall time from contract authoring to val_loss=9.3837: ~14 hours.

## Training curve highlights

- epoch 0: train=9.7567, val=10.0698 (init)
- epoch 10: train=9.4610, val=9.5657 (post-warmup)
- epoch 30: train=9.2x, val=9.42x
- epoch 44: val=9.3837 (BEST)
- epoch 50: train=9.2093, val=9.3889 (EARLY_STOP next)

Full per-epoch metadata in evidence/model-2-codeparrot-retrain-2026-04-28/all-epochs.json.

## Coverage impact

§33 is binding evidence for SHIP-021 (corpus diversity binding) — promotion
to DISCHARGED is deferred to a separate PR that updates the SHIP-021
contract atomically. Spec scoreboard unchanged (15+33) in this PR.

## Files

- evidence/model-2-codeparrot-retrain-2026-04-28/launch.log
- evidence/model-2-codeparrot-retrain-2026-04-28/all-epochs.json
- §33 spec section (8 subsections, ~80 lines)
- Header: v2.77.0 → v2.78.0

## Methodology landed

The §26.8 stack-tool-extension rule paid off concretely:
- 6h authoring cost (P1.0 contract + P1.1 impl) → permanent apr capability
- Every future dataset pull benefits
- §33's val_loss=9.3837 is downstream proof of the methodology

This commit represents the first cycle in §22→§33 where the spec amendment
has the same priority as the empirical result.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>