feat(retrieval): add part-number discriminator and improve part disambiguation.

inference.py

@@ -56,6 +56,7 @@ _SHORT_CHUNK_PENALTY = 0.15
 
 _GRADE_RE = re.compile(r"\b(33|43|53)\b")
 _PART_BASE_RE = re.compile(r"IS\s+(\d+)\s*\(", re.IGNORECASE)
+_PART_NUM_RE = re.compile(r"\bpart[\s\-–—]*(\d+)\b", re.IGNORECASE)
 _STOP_WORDS = {"and", "or", "for", "the", "of", "in", "a", "an", "to"}
 
 
@@ -212,6 +213,9 @@ class Retriever:
         grade_in_query = set(_GRADE_RE.findall(query))
         sig_query = query_words - _STOP_WORDS
 
+        # Detect explicit part number in query (e.g. "Part 2", "Part 1")
+        part_nums_in_query = set(_PART_NUM_RE.findall(query))
+
         for cid, base in list(chunk_scores.items()):
             chunk = self.idx.chunks[cid]
             sid_norm = _norm_std_id(chunk["standard_id"])
@@ -257,6 +261,25 @@ class Retriever:
                 elif grade_in_title and grade_in_title.isdisjoint(grade_in_query):
                     bonus -= 0.40
 
+            # Part-number discriminator: when query explicitly names a part number,
+            # boost matching parts and penalise non-matching siblings.
+            if part_nums_in_query:
+                sid_raw = chunk["standard_id"]
+                m_part = re.search(r"\(Part\s*(\d+)\)", sid_raw, re.IGNORECASE)
+                if m_part:
+                    part_num = m_part.group(1)
+                    if part_num in part_nums_in_query:
+                        bonus += 0.30
+                    else:
+                        bonus -= 0.20
+                else:
+                    # Base standard (no Part N) — if query says "Part 1 General"
+                    # treat as matching (base IS is often the Part 1/General Req)
+                    if "1" in part_nums_in_query and (
+                        "general" in query_lower or "requirement" in query_lower
+                    ):
+                        bonus += 0.20
+
             chunk_scores[cid] = base + bonus
 
         # --- Group by standard_id, keep best chunk score ---
@@ -272,11 +295,14 @@ class Retriever:
         # --- Same-family Part disambiguation ---
         base_to_sids: dict[str, list[str]] = {}
         for sid in std_best:
-            if "(" not in sid:
-                continue
             m = _PART_BASE_RE.match(sid)
             if m:
                 base_to_sids.setdefault(m.group(1), []).append(sid)
+            elif re.match(r"IS\s+\d+\s*:", sid, re.IGNORECASE):
+                # Base standard (no Part N) — register under its IS number
+                m2 = re.match(r"IS\s+(\d+)", sid, re.IGNORECASE)
+                if m2:
+                    base_to_sids.setdefault(m2.group(1), []).append(sid)
         for sids in base_to_sids.values():
             if len(sids) < 2:
                 continue