fix(fb-39): address PR #136 round 1 review

kebab eval compare now surfaces precision_at_k_chunk delta in both
human-readable table + deltas JSON. Snapshot fixture regenerated
additively.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-eval/tests/fixtures/eval/compare-1.json

@@ -65,6 +65,12 @@
       "5": 0.33329999446868896
     },
     "mrr": 0.41659998893737793,
+    "precision_at_k_chunk": {
+      "1": 0.33340001106262207,
+      "10": 0.0333000048995018,
+      "3": 0.22219999134540558,
+      "5": 0.06669999659061432
+    },
     "recall_at_k_doc": {
       "1": 0.33340001106262207,
       "10": 0.33329999446868896,