Add recomputable fingerprint to contamination number (idx_sha + canonical recipe + method/params-status) per as_metoda-badawcza §5a

README.md

@@ -94,6 +94,12 @@ questions (1,662 / 18,820) still have variants/paraphrases in the corpus** — a
 in `web` (1,653) vs curated (9). The build-time 8-gram filter missed reworded/partial variants
 that the longer fragment match catches.
 
+**Recomputable fingerprint (verify by recompute, not re-read):**
+- `idx_sha` = `a2e4299364f999133e03989230d8730388d804e433f6a190d60d6b1208e079ce`
+- recipe: `sha256(json.dumps(sorted(contaminated_item_idx), separators=(",",":")).encode("utf-8"))` — the canonical sha of the sorted index list, **not** a file hash (robust to metadata/wrapper edits; the sha points at the thing, not its packaging).
+- number: `len(idx) = 1662`; `1662 / 18820 = 8.83%`; clean `18820 − 1662 = 17158`. Anyone can re-derive every figure here from `idx_sha` — no metadata re-read.
+- method: 12-word contiguous fragment match (Aho-Corasick) vs the full 31 GB corpus. `params: not-preserved (pre-norm)` — the scan script/params were not retained, so the method is **documentational, not re-runnable**; the *number* stays recomputable from `idx_sha`, but re-deriving the index from scratch would need a fresh scan.
+
 **Implication:** any benchmark built on **LLMzSzŁ** — and likely other publicly-published
 Polish state exams (matura, egzamin ósmoklasisty, egzaminy zawodowe) — will be **inflated**
 for models trained on this corpus, because the model saw ~8.83% of the items. This is inherent to