fix(发际线): 修正不对称刘海导致的模板叠加倾斜

按 MediaPipe 左右对应额头点协调模板延伸距离，保留人脸姿态与头部轮廓钳制，统一预览白线和生发遮罩的几何。补充对齐回归测试并更新贴图顺序测试及实现说明。

验证：13 项相关测试通过；两张用户图片完成前后对比，新图男性 7 型接口与生发图均成功返回。

hairline/hairline_2d.py

@@ -124,6 +124,46 @@ def build_hairline_mask(parse_map: np.ndarray) -> np.ndarray:
     return build_hair_mask(parse_map)
 
 
+def align_hairline_for_template(
+    landmarks_norm: np.ndarray,
+    hairline_norm: np.ndarray,
+    valid: np.ndarray,
+    image_shape: tuple[int, int],
+) -> np.ndarray:
+    """Keep a designed template aligned with the face rather than the fringe.
+
+    Uneven bangs can make the detected hair boundary much higher on one
+    side, shearing an otherwise balanced template. Average the upward
+    extension of each anatomical anchor pair, measured in image pixels.
+    Keep the face anchors and the perpendicular component unchanged so
+    facial roll/perspective remain visible; do not level the curve in the
+    image. Missing ray hits retain their fallback and never influence a
+    valid partner. Call silhouette clamping afterwards to bound the result.
+
+    This is for template placement only, not measurement of the real hairline.
+    """
+    h, w = image_shape
+    scale = np.array([w, h], dtype=np.float64)
+    up = face_up_vector(landmarks_norm) * scale
+    length = np.linalg.norm(up)
+    if length < 1e-9:
+        return hairline_norm.copy()
+    up /= length
+
+    anchors = landmarks_norm[C.MP_TOP_ANCHORS, :2] * scale
+    points = hairline_norm * scale
+    distances = (points - anchors) @ up
+    indices = {mp_idx: i for i, mp_idx in enumerate(C.MP_TOP_ANCHORS)}
+    for left, right in C.MP_TOP_ANCHOR_PAIRS:
+        i, j = indices[left], indices[right]
+        if not (valid[i] and valid[j]):
+            continue
+        mean_distance = (distances[i] + distances[j]) / 2.0
+        for k in (i, j):
+            points[k] += (mean_distance - distances[k]) * up
+    return (points / scale).astype(np.float32)
+
+
 def smooth_hairline(
     hairline_norm: np.ndarray,
     valid: np.ndarray,