Fix chrF fallback so English wiki garbage fails translation_success

Sacrebleu errors used to fall back to set-overlap unigrams, so any
English hyp scored ~70–80 against English refs and SFT reported
success 1.0. Use count-based char n-grams, keep BLEU failures from
clobbering chrF, and print a few hyps during eval.

kda/training/success.py

@@ -28,8 +28,33 @@ def _detect_lang(text: str) -> str | None:
     return tag[:2]
 
 
+def _chrf_ngram(hyp: str, ref: str, max_n: int = 4) -> float:
+    """Count-based char n-gram F (β=2), 0–100. Not set-overlap unigrams."""
+    from collections import Counter
+
+    hyp, ref = hyp.strip(), ref.strip()
+    if not hyp or not ref:
+        return 0.0
+    scores: list[float] = []
+    for n in range(1, max_n + 1):
+        if len(hyp) < n or len(ref) < n:
+            scores.append(0.0)
+            continue
+        hc = Counter(hyp[i : i + n] for i in range(len(hyp) - n + 1))
+        rc = Counter(ref[i : i + n] for i in range(len(ref) - n + 1))
+        overlap = sum((hc & rc).values())
+        prec = overlap / max(sum(hc.values()), 1)
+        rec = overlap / max(sum(rc.values()), 1)
+        if prec + rec == 0:
+            scores.append(0.0)
+            continue
+        beta2 = 4.0
+        scores.append((1.0 + beta2) * prec * rec / (beta2 * prec + rec))
+    return 100.0 * sum(scores) / max(len(scores), 1)
+
+
 def _chrf(hyp: str, ref: str) -> float:
-    """chrF++ in 0–100. Falls back to char unigram F if sacrebleu is missing."""
+    """chrF++ in 0–100. Falls back to count-based char n-grams if sacrebleu is missing."""
     if not hyp or not ref:
         return 0.0
     try:
@@ -37,16 +62,7 @@ def _chrf(hyp: str, ref: str) -> float:
 
         return float(CHRF(word_order=2).sentence_score(hyp, [ref]).score)
     except Exception:
-        hyp_c, ref_c = list(hyp), list(ref)
-        if not hyp_c:
-            return 0.0
-        ref_set = set(ref_c)
-        overlap = sum(1 for c in hyp_c if c in ref_set)
-        prec = overlap / len(hyp_c)
-        rec = overlap / max(len(ref_c), 1)
-        if prec + rec == 0:
-            return 0.0
-        return 100.0 * 2 * prec * rec / (prec + rec)
+        return _chrf_ngram(hyp, ref)
 
 
 def translation_success(