Add model card: what this is, and which variants are valid

README.md

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+---
+library_name: transformers
+tags:
+- model-merging
+- mergeability
+- research-artifact
+- do-not-use-aligned-variant
+---
+
+# polypythia-14m-s1s2
+
+Merged checkpoints for one model pair. **6 variant(s)** in this repository: `average__aligned`, `average__naive`, `task_arithmetic__aligned`, `task_arithmetic__naive`, `ties__aligned`, `ties__naive`.
+
+- Operators present: `average`, `task_arithmetic`, `ties`
+- Arms present: `aligned`, `naive`
+
+## ⚠️ The `__aligned` variants in this repository are not usable
+
+They were merged from models damaged by a bug in our alignment code. The role table that classifies a
+parameter by its relation to the residual stream did not recognise this architecture's tensor names,
+so **90.8% of the residual-facing parameter mass was left unpermuted** while attention and both norm
+tensors were permuted. The result is not an aligned model; it is a broken one. Under a map that must
+be exactly function-preserving, the logits moved by a relative 0.32 (it must be ~1e-7).
+
+**Do not use, cite, or benchmark against the `__aligned` folders here.** The `__naive` and
+`__transport` folders are unaffected and remain valid: the bug only ever touched the aligned arm.
+
+The bug is fixed (`common/alignment.py` now classifies these names, `residual_coverage()` reports the
+fraction classified, and `align_state_dict(..., strict=True)` raises rather than silently skipping
+tensors when it cannot classify ≥99% of the residual-facing mass). Corrected aligned merges are being
+regenerated from the re-run sweep.
+
+
+## What this is
+
+A **model-merging artefact**, not a model anyone should deploy. Each folder is one merge of two parent
+models, published so that the merging results in the accompanying work are inspectable and
+reproducible rather than taken on trust.
+
+Folders are named `<recipe>__<arm>`:
+
+| part | meaning |
+|---|---|
+| `average`, `task_arithmetic`, `ties`, `qmd_guided` | the merge operator |
+| `__naive` | parents merged as-is |
+| `__aligned` | parents merged after a function-preserving alignment map |
+| `__transport` | parents merged after vocabulary transport (cross-tokenizer pairs) |
+
+## Regime warning, which matters more than usual here
+
+These are **sub-1B models pretrained from scratch**, not fine-tunes of a shared checkpoint. In this
+regime merging is *destructive*: across 2,320 merges the mean change in NLL against the better parent
+is about **-1.4 nats**, and a merge that beats its better parent is rare. That is a finding, not a
+defect of these artefacts, and it is the opposite of what the model-soup literature reports for the
+regime it studies. Both can be true; the regimes differ. Do not read a merge here as a capable model.
+
+## Provenance of the numbers
+
+Merge quality for every artefact in this org is recorded in the accompanying repository's sweep
+tables, with `delta_floor` (nats against the better parent) as the outcome. Alignment-dependent
+columns from sweeps predating the fix are void; see the note above where it applies.