feat: 接口1/6 发际线贴近头顶时改为修正而非弃用置null

顶庭(头顶→发际线)<0.7cm 时不再返回 null：
- 上庭 = 0.95×中庭 反推发际线；若越过头顶则顶庭固定 0.7cm（发际线钉在头顶下方）
- hairline_source 新增 "corrected"；to_response 始终输出完整四庭
- 标注图始终正常画发际线横线与庭段
- 接口6 三庭逻辑同步，去 discarded 分支

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

tests/test_api.py

@@ -173,7 +173,7 @@ def test_hairline_gen_female(client, monkeypatch):
     assert set([f"eye{i}" for i in range(1, 8)]).issubset(fm["seven_eyes"].keys())
     assert set(["hair_top", "hairline", "brow_center",
                 "nose_bottom", "chin_tip"]).issubset(fm["landmarks"].keys())
-    assert fm["hairline_source"] in ("segmentation", "estimated")
+    assert fm["hairline_source"] in ("segmentation", "estimated", "corrected")
     assert set(["yaw", "pitch", "roll"]).issubset(fm["head_pose"].keys())
 
 
@@ -200,7 +200,7 @@ def test_success_structure(client):
     assert data["seven_eyes"]["eye3"] is not None
     assert data["seven_eyes"]["eye4"] is not None
     assert data["seven_eyes"]["eye5"] is not None
-    assert data["hairline_source"] in ("segmentation", "estimated")
+    assert data["hairline_source"] in ("segmentation", "estimated", "corrected")
     # base64 解码为合法 PNG（非 URL）
     assert "annotated_image_url" not in data
     png = base64.b64decode(data["annotated_image_base64"])