S5-5: centralized part-box synthesis (band-y x content-margins)

Add synthesize_part_box() as the single authoritative S5 part-box projection
(T3 swap point): content-margin x-extent x part-band y-extent, BOTTOMLEFT
coords; label_box retained as a separate anchor. build() attaches box per part.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

api/services/docling/template.py

@@ -44,6 +44,34 @@ def _furn_kind(it):
     return "chrome_text"
 
 
+def synthesize_part_box(part_band, content_x_band):
+    """Return the one authoritative S5 part-box projection.
+
+    Parts remain boxes in S5, but the box is a projection rather than intrinsic
+    geometry: document content margins provide the x-extent and the part band
+    provides y. The band end is already bounded by the next part in bands.py;
+    the original label box remains a separate anchor for rendering/review.
+
+    Coordinates stay in the first-pass PDF-point BOTTOMLEFT bbox shape.
+    """
+    if not content_x_band:
+        return None
+    try:
+        x_left = content_x_band["x_left"]
+        x_right = content_x_band["x_right"]
+        y_start = part_band["y_start"]
+        y_end = part_band["y_end"]
+    except KeyError:
+        return None
+    return {
+        "l": round(x_left, 1),
+        "t": round(y_start, 1),
+        "r": round(x_right, 1),
+        "b": round(y_end, 1),
+        "coord_origin": "BOTTOMLEFT",
+    }
+
+
 def build(structured, bands, furniture, pdf=None, page_roles=None):
     page_roles = page_roles or {}
     part_bbox = {p["label"]: p.get("bbox")
@@ -124,10 +152,15 @@ def build(structured, bands, furniture, pdf=None, page_roles=None):
         main = [{"question": m["question"], "y_start": m["y_start"], "y_end": m["y_end"],
                  "is_start": m.get("is_start", True),
                  "source": "auto", "confirmed": False} for m in pb["main"]]
-        part = [{"label": p["label"], "question": p["question"],
-                 "y_start": p["y_start"], "y_end": p["y_end"],
-                 "label_box": part_bbox.get(p["label"]),     # app may render a box instead of lines
-                 "source": "auto", "confirmed": False} for p in pb["part"]]
+        part = []
+        for p in pb["part"]:
+            part.append({
+                "label": p["label"], "question": p["question"],
+                "y_start": p["y_start"], "y_end": p["y_end"],
+                "label_box": part_bbox.get(p["label"]),  # anchor, not the part extent
+                "box": synthesize_part_box(p, xband),
+                "source": "auto", "confirmed": False,
+            })
         pr = page_roles.get(pgs) or page_roles.get(pg) or {}
         pages[pgs] = {
             "role": pr.get("role", "question"),