test: _measure_coverage overall 排除零内容维度 - Closes #36

添加 3 个回归测试验证 total=0 的维度不参与 overall 计算：
- 零内容维度被正确排除
- 所有维度有内容则全部参与
- 无内容时返回 0.0
fix 已在 1a867b0 合入，本次补充 UT 覆盖。

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

tests/acceptance/test_main_health.py

@@ -291,6 +291,85 @@ def _measure_coverage(ir_data: dict, parsed_data: dict) -> dict:
     }
 
 
+def test_measure_coverage_excludes_zero_dimensions():
+    """#36: dimensions with total=0 must not drag down the overall rate.
+
+    When diagram total=0, the overall should be computed from sections and tables
+    only, not include a 0% diagram entry that makes the goal unreachable.
+    """
+    parsed_data = {
+        "sections": [
+            {"source": "3.1.1 功能A", "blocks": [
+                {"type": "table", "rows": [{"cell": "1"}, {"cell": "2"}]}
+            ]}
+        ],
+        "image_analysis": [],  # no diagrams → total=0
+    }
+    # IR that covers the section but no table rows (table coverage = 0/2)
+    ir_data = {
+        "rules": [
+            {"sources": [{"section": "3.1.1"}]}  # 1 section covered, 0 tables
+        ]
+    }
+
+    cov = _measure_coverage(ir_data, parsed_data)
+
+    # Section: 1/1 = 100%, Table: 0/2 = 0%, Diagram: total=0 → excluded
+    assert cov["section_coverage"]["total"] == 1
+    assert cov["section_coverage"]["rate"] == 1.0
+    assert cov["table_coverage"]["total_rows"] == 2
+    assert cov["table_coverage"]["rate"] == 0.0
+    assert cov["diagram_coverage"]["total"] == 0
+    assert cov["diagram_coverage"]["rate"] == 1.0  # _safe_rate: 0/0 → 1.0
+
+    # Key assertion: diagram (total=0) is excluded from overall
+    # overall = (1.0 + 0.0) / 2 = 0.5
+    # NOT (1.0 + 0.0 + 1.0) / 3 = 0.667
+    assert cov["overall_rate"] == 0.5, (
+        f"Expected overall 0.5 (sections + tables only), got {cov['overall_rate']}. "
+        f"Zero-content dimension may be leaking into the average."
+    )
+
+
+def test_measure_coverage_all_dimensions_have_content():
+    """When all dimensions have content, all should be included."""
+    parsed_data = {
+        "sections": [
+            {"source": "3.1.1 功能A", "blocks": [
+                {"type": "table", "rows": [{"cell": "1"}]}
+            ]}
+        ],
+        "image_analysis": [{"type": "flowchart", "rid": "img_001"}],
+    }
+    ir_data = {
+        "rules": [
+            {"sources": [{"section": "3.1.1"}]},
+            {"sources": [{"type": "table", "section": "3.1.1", "row": 0}]},
+            {"sources": [{"type": "logic_tree", "image_id": "img_001"}]},
+        ]
+    }
+
+    cov = _measure_coverage(ir_data, parsed_data)
+
+    # All three dimensions have content → all included
+    assert cov["section_coverage"]["total"] == 1
+    assert cov["table_coverage"]["total_rows"] == 1
+    assert cov["diagram_coverage"]["total"] == 1
+    # overall = (1.0 + 1.0 + 1.0) / 3 = 1.0
+    assert cov["overall_rate"] == 1.0, (
+        f"Expected overall 1.0 (all covered), got {cov['overall_rate']}"
+    )
+
+
+def test_measure_coverage_no_content_returns_zero():
+    """When no dimensions have content, overall should be 0.0."""
+    parsed_data = {"sections": [], "image_analysis": []}
+    ir_data = {"rules": []}
+
+    cov = _measure_coverage(ir_data, parsed_data)
+    assert cov["overall_rate"] == 0.0
+
+
 def test_layer_b_coverage(
     ir_data: dict,
     parsed_data: dict | None,