fix: _measure_coverage 零内容维度不再拉低 overall 覆盖率 - Closes #21

当某个维度（如图表）无内容时（total=0），rate 设为 1.0 且不参与 overall 均分。
此前 0/0 被算作 0%，将 overall 从 86.1% 拉低到 57.4%。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/acceptance/test_main_health.py

@@ -227,10 +227,14 @@ def _measure_coverage(ir_data: dict, parsed_data: dict) -> dict:
                 if matched:
                     covered_sections.add(matched)
 
+    def _safe_rate(covered: int, total: int) -> float:
+        """Return coverage rate. total=0 means nothing to cover → 1.0."""
+        return round(covered / total, 3) if total > 0 else 1.0
+
     section_coverage = {
         "total": len(func_sections),
         "covered": len(covered_sections),
-        "rate": round(len(covered_sections) / max(len(func_sections), 1), 3),
+        "rate": _safe_rate(len(covered_sections), len(func_sections)),
         "uncovered": [s["name"] for s in func_sections
                       if s["name"] not in covered_sections],
     }
@@ -249,7 +253,7 @@ def _measure_coverage(ir_data: dict, parsed_data: dict) -> dict:
     table_coverage = {
         "total_rows": total_rows,
         "covered_rows": len(covered_rows),
-        "rate": round(len(covered_rows) / max(total_rows, 1), 3),
+        "rate": _safe_rate(len(covered_rows), total_rows),
     }
 
     # ── diagram coverage ──
@@ -265,16 +269,18 @@ def _measure_coverage(ir_data: dict, parsed_data: dict) -> dict:
     diagram_coverage = {
         "total": len(diagram_rids),
         "covered": len(covered_rids),
-        "rate": round(len(covered_rids) / max(len(diagram_rids), 1), 3),
+        "rate": _safe_rate(len(covered_rids), len(diagram_rids)),
         "uncovered": [r for r in diagram_rids if r not in covered_rids],
     }
 
-    # ── overall ──
-    rates = [
-        section_coverage["rate"],
-        table_coverage["rate"],
-        diagram_coverage["rate"],
-    ]
+    # ── overall: only include dimensions with actual content ──
+    rates: list[float] = []
+    if section_coverage["total"] > 0:
+        rates.append(section_coverage["rate"])
+    if table_coverage["total_rows"] > 0:
+        rates.append(table_coverage["rate"])
+    if diagram_coverage["total"] > 0:
+        rates.append(diagram_coverage["rate"])
     overall = round(sum(rates) / len(rates), 3) if rates else 0.0
 
     return {