feat(接口2): 移植head3d发际线管线 + 接口2实现方案文档

- 从head3d复制发际线检测管线到 hairline/ 包：MediaPipe Tasks + SegFormer分割
  + 17锚点射线检测 + 502点mesh(face_ext.obj)+UV
- 复制模型：face_landmarker.task(3.7MB)、SegFormer config/preprocessor
  (model.safetensors 340MB 单独下载中)
- 新增 docs/接口2-C端生发-技术实现方案.md：第一步=发际线曲线叠加预览图，
  新增gender必填参数，按性别贴图数量输出(female5/male4)，hairline_type英文key，
  服务端cv2逐三角形warp渲染器(head3d只有浏览器端Three.js渲染)
- 接口文档.md 接口2章节同步：gender参数、输出语义、错误码说明
- hairline_texture/ 9张发际线贴图入库

hairline/obj_io.py

@@ -0,0 +1,97 @@
+"""Minimal Wavefront OBJ reader/writer.
+
+Tailored to the project's face.obj:
+- Latin-1 / GB-encoded comments allowed (we tolerate undecodable bytes).
+- Vertices written with 4 decimal places (matches existing file).
+- Preserves comment/mtllib/group lines if requested.
+"""
+from __future__ import annotations
+from dataclasses import dataclass, field
+from typing import Sequence
+
+
+@dataclass
+class ObjMesh:
+    """1-indexed in OBJ files; stored 0-indexed internally."""
+    positions: list[tuple[float, float, float]] = field(default_factory=list)
+    texcoords: list[tuple[float, float]]        = field(default_factory=list)
+    normals:   list[tuple[float, float, float]] = field(default_factory=list)
+    # Each face is a list of 3 (pos_idx, uv_idx, normal_idx) tuples, 0-indexed.
+    # -1 means "absent".
+    faces:     list[list[tuple[int, int, int]]] = field(default_factory=list)
+    header_lines: list[str] = field(default_factory=list)  # comments / mtllib
+
+    def n_v(self) -> int:  return len(self.positions)
+    def n_vt(self) -> int: return len(self.texcoords)
+    def n_vn(self) -> int: return len(self.normals)
+    def n_f(self) -> int:  return len(self.faces)
+
+
+def read_obj(path: str) -> ObjMesh:
+    """Read an OBJ file, ignoring undecodable bytes in comments."""
+    with open(path, "rb") as f:
+        raw = f.read()
+    text = raw.decode("latin-1", errors="replace")
+    mesh = ObjMesh()
+
+    for line in text.splitlines():
+        line = line.strip()
+        if not line:
+            continue
+        if line.startswith("#") or line.startswith("mtllib") or line.startswith("o ") or line.startswith("g ") or line.startswith("s "):
+            mesh.header_lines.append(line)
+            continue
+        parts = line.split()
+        kind = parts[0]
+        if kind == "v":
+            mesh.positions.append((float(parts[1]), float(parts[2]), float(parts[3])))
+        elif kind == "vt":
+            mesh.texcoords.append((float(parts[1]), float(parts[2])))
+        elif kind == "vn":
+            mesh.normals.append((float(parts[1]), float(parts[2]), float(parts[3])))
+        elif kind == "f":
+            verts = []
+            for spec in parts[1:]:
+                seg = spec.split("/")
+                pi = int(seg[0]) - 1 if seg[0] else -1
+                ti = int(seg[1]) - 1 if len(seg) > 1 and seg[1] else -1
+                ni = int(seg[2]) - 1 if len(seg) > 2 and seg[2] else -1
+                verts.append((pi, ti, ni))
+            # Triangulate fan if quad/n-gon (shouldn't happen for our mesh)
+            for k in range(1, len(verts) - 1):
+                mesh.faces.append([verts[0], verts[k], verts[k + 1]])
+        # ignore everything else
+    return mesh
+
+
+def write_obj(path: str, mesh: ObjMesh, header: Sequence[str] | None = None) -> None:
+    """Write an OBJ file with the project's conventions (1-indexed, 4dp)."""
+    out: list[str] = []
+    if header is not None:
+        out.extend(header)
+    else:
+        out.append("# head3d extended face mesh")
+        out.append(f"# vertices={mesh.n_v()} texcoords={mesh.n_vt()} normals={mesh.n_vn()} faces={mesh.n_f()}")
+
+    out.append("")
+    for x, y, z in mesh.positions:
+        out.append(f"v  {x:.4f} {y:.4f} {z:.4f}")
+    out.append("")
+    for u, v in mesh.texcoords:
+        out.append(f"vt {u:.4f} {v:.4f} 0.0000")
+    out.append("")
+    for nx, ny, nz in mesh.normals:
+        out.append(f"vn {nx:.4f} {ny:.4f} {nz:.4f}")
+    out.append("")
+    for face in mesh.faces:
+        toks = []
+        for pi, ti, ni in face:
+            a = str(pi + 1)
+            b = str(ti + 1) if ti >= 0 else ""
+            c = str(ni + 1) if ni >= 0 else ""
+            toks.append(f"{a}/{b}/{c}")
+        out.append("f " + " ".join(toks))
+
+    with open(path, "w", encoding="utf-8", newline="\r\n") as f:
+        f.write("\n".join(out))
+        f.write("\n")