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/src/compare.rs

@@ -184,6 +184,18 @@ pub fn render_report_md(report: &CompareReport) -> String {
             ),
         );
     }
+    for k in crate::metrics::TOP_K_VARIANTS {
+        let _ = writeln!(
+            out,
+            "| precision@{k}_chunk | {} | {} | {} |",
+            fmt(a.precision_at_k_chunk.get(k).copied().unwrap_or(f32::NAN)),
+            fmt(b.precision_at_k_chunk.get(k).copied().unwrap_or(f32::NAN)),
+            fmt_delta(
+                a.precision_at_k_chunk.get(k).copied().unwrap_or(f32::NAN),
+                b.precision_at_k_chunk.get(k).copied().unwrap_or(f32::NAN),
+            ),
+        );
+    }
     let _ = writeln!(
         out,
         "| citation_coverage | {} | {} | {} |",
@@ -419,6 +431,7 @@ fn build_deltas(
     }
     let mut hit = serde_json::Map::new();
     let mut recall = serde_json::Map::new();
+    let mut precision = serde_json::Map::new();
     for k in crate::metrics::TOP_K_VARIANTS {
         hit.insert(
             k.to_string(),
@@ -434,11 +447,19 @@ fn build_deltas(
                 b.recall_at_k_doc.get(k).copied().unwrap_or(f32::NAN),
             ),
         );
+        precision.insert(
+            k.to_string(),
+            d(
+                a.precision_at_k_chunk.get(k).copied().unwrap_or(f32::NAN),
+                b.precision_at_k_chunk.get(k).copied().unwrap_or(f32::NAN),
+            ),
+        );
     }
     serde_json::json!({
         "hit_at_k": hit,
         "mrr": d(a.mrr, b.mrr),
         "recall_at_k_doc": recall,
+        "precision_at_k_chunk": precision,
         "citation_coverage": d(a.citation_coverage, b.citation_coverage),
         "groundedness": d(a.groundedness, b.groundedness),
         "empty_result_rate": d(a.empty_result_rate, b.empty_result_rate),

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,