docs(logo): 新增 logo v5.1 正式资产、使用规范与构建脚本

- 白文印「标」（AI 生成原图经脚本抠图、统一为 #D9291C、矢量化）
- 字标：思源宋体 SemiBold + Libre Caslon；比例 H2，英文视觉内缩 C2
- svg/、png/、favicon：横版、竖版、反白、单色、印章、各尺寸图标
- scripts/：logo_lib.py 统一排版参数，build_assets.sh 一键生成
- logo-spec.md、logo-sheet.html、字体许可证（OFL）

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/design/logo/scripts/process_seal.py

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+"""把位图生成的印章处理成 logo 资产：抠图、统一颜色、矢量化、小尺寸版本。
+
+用法：.work/venv/bin/python scripts/process_seal.py <源图> <输出目录> [品牌红 HEX]
+"""
+import sys
+from pathlib import Path
+
+import numpy as np
+import potrace
+from PIL import Image
+from scipy import ndimage
+
+src, out_dir = Path(sys.argv[1]), Path(sys.argv[2])
+brand = sys.argv[3] if len(sys.argv) > 3 else "#D9291C"
+out_dir.mkdir(parents=True, exist_ok=True)
+rgb = tuple(int(brand[i:i + 2], 16) for i in (1, 3, 5))
+
+im = np.asarray(Image.open(src).convert("RGB")).astype(float)
+redness = (im[..., 0] - (im[..., 1] + im[..., 2]) / 2) / 255
+
+# 印泥浓淡 → 透明度：保留颗粒和毛边，背景完全透明
+alpha = np.clip((redness - 0.12) / (0.55 - 0.12), 0, 1)
+alpha = alpha * alpha * (3 - 2 * alpha)  # smoothstep，让颗粒边缘柔和
+
+ys, xs = np.where(redness > 0.35)
+y0, y1, x0, x1 = ys.min(), ys.max() + 1, xs.min(), xs.max() + 1
+side = max(y1 - y0, x1 - x0)
+pad = round(side * 0.02)
+canvas = side + 2 * pad
+cy, cx = (y0 + y1) // 2, (x0 + x1) // 2
+half = canvas // 2
+
+
+def crop(a):
+    out = np.zeros((canvas, canvas), dtype=a.dtype)
+    sy0, sx0 = cy - half, cx - half
+    ys_, xs_ = slice(max(sy0, 0), min(sy0 + canvas, a.shape[0])), slice(max(sx0, 0), min(sx0 + canvas, a.shape[1]))
+    out[ys_.start - sy0:ys_.stop - sy0, xs_.start - sx0:xs_.stop - sx0] = a[ys_, xs_]
+    return out
+
+
+alpha_c = crop(alpha)
+red_c = crop(redness)
+
+# 1. 质感主图（透明 PNG）
+rgba = np.zeros((canvas, canvas, 4), dtype=np.uint8)
+rgba[..., :3] = rgb
+rgba[..., 3] = (alpha_c * 255).round().astype(np.uint8)
+Image.fromarray(rgba).save(out_dir / "seal-textured.png")
+on_white = np.full((canvas, canvas, 3), 255.0)
+on_white = on_white * (1 - alpha_c[..., None]) + np.array(rgb) * alpha_c[..., None]
+Image.fromarray(on_white.round().astype(np.uint8)).save(out_dir / "seal-textured-on-white.png")
+
+
+def trace(mask, name, turdsize, alphamax=1.0, fill=brand):
+    bm = potrace.Bitmap(~mask)  # potracer 把 False 当作前景，这里取反
+    path = bm.trace(turdsize=turdsize, alphamax=alphamax, opticurve=True, opttolerance=0.2)
+    parts = []
+    for curve in path:
+        s = curve.start_point
+        d = [f"M{s.x:.1f} {s.y:.1f}"]
+        for seg in curve.segments:
+            if seg.is_corner:
+                d.append(f"L{seg.c.x:.1f} {seg.c.y:.1f}L{seg.end_point.x:.1f} {seg.end_point.y:.1f}")
+            else:
+                d.append(f"C{seg.c1.x:.1f} {seg.c1.y:.1f} {seg.c2.x:.1f} {seg.c2.y:.1f} {seg.end_point.x:.1f} {seg.end_point.y:.1f}")
+        d.append("Z")
+        parts.append("".join(d))
+    svg = (f'<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 {canvas} {canvas}">'
+           f'<path fill="{fill}" fill-rule="evenodd" d="{"".join(parts)}"/></svg>')
+    (out_dir / name).write_text(svg)
+    return len(svg)
+
+
+# 2. 质感矢量版：放大 2 倍再描摹，保留颗粒
+big = np.asarray(Image.fromarray((alpha_c * 255).astype(np.uint8)).resize((canvas * 2, canvas * 2), Image.LANCZOS)) / 255
+n_tex = trace(big > 0.5, "seal-textured-2x.svg", turdsize=6)
+# 描摹坐标是 2 倍的，修正 viewBox
+p = out_dir / "seal-textured-2x.svg"
+p.write_text(p.read_text().replace(f'viewBox="0 0 {canvas} {canvas}"', f'viewBox="0 0 {canvas * 2} {canvas * 2}"'))
+p.rename(out_dir / "seal-textured.svg")
+
+# 3. 干净版：先模糊去掉颗粒，再二值化描摹，得到平滑的印面和字形
+solid = ndimage.gaussian_filter((red_c > 0.35).astype(float), sigma=canvas * 0.006) > 0.5
+solid = ndimage.binary_fill_holes(solid) & solid  # 保留白字（它们与外部不连通时是「洞」）
+solid = ndimage.binary_opening(solid, iterations=2)
+n_clean = trace(solid, "seal-clean.svg", turdsize=canvas * 0.0005 * canvas, alphamax=1.1)
+
+# 4. 小尺寸版：白色笔画加粗（墨色腐蚀），保证 32px 下笔画不消失
+thick = ndimage.binary_erosion(solid, iterations=max(2, round(canvas * 0.012)))
+n_small = trace(thick, "seal-small.svg", turdsize=canvas * 0.0008 * canvas, alphamax=1.1)
+
+# 5. 纸底：印面外轮廓（不含白字），向内收一点，深色背景上垫在印章下面，让白字和颗粒显示为纸色
+paper = ndimage.binary_erosion(ndimage.binary_fill_holes(solid), iterations=max(2, round(canvas * 0.006)))
+trace(paper, "seal-paper.svg", turdsize=canvas * 0.001 * canvas, alphamax=1.2, fill="#FFFFFF")
+
+print(f"canvas={canvas}px brand={brand} textured.svg={n_tex // 1024}KB clean.svg={n_clean // 1024}KB small.svg={n_small // 1024}KB")