fix: 树在 Cesium 远看发黑 — 补 emissive 并导出切线

黑色色块修掉后仍存在：近看正常，远看整棵树是暗色块。逐项排除：

- 贴图颜色全链路无偏移，源 4k / 降采样 2k / GLB 内 PNG 的 opaque 均值
  都是 sRGB [0.409, 0.406, 0.362]，不是 gamma 问题。
- 模拟 GPU mip 链，可见像素亮度 mip0→mip6 只从 0.127 变到 0.124，
  覆盖率稳定 0.42，不是 mipmap 变暗。
- 法线贴图抠图区域是干净的平面法线 [0.494, 0.512, 0.988]，无黑像素。

真实原因是缺 emissive 补偿。预览页 skyBox / skyAtmosphere / sun 全部关闭
且未配置环境贴图，Cesium 只剩很弱的默认球谐环境光，太阳直射不到的面接近
全黑——这正是 EXPORT_EMISSION_OVERRIDES 存在的原因（建筑 0.18，程序化树冠
0.015~0.02）。树冠绝大部分是背光叶片卡片，远看塌成一团暗色，近看能看到
向阳面所以还行。apple 材质不在那张表里。

- 抠图植被把 diffuse 接回 Emission Color，FOLIAGE_EMISSION = 0.22。
  用带贴图的自发光而非平坦颜色：常量会把树干染成叶子绿，而这样每个像素
  下限是自身反照率的一个比例。不增加字节，emissiveTexture 指向 base color
  已在用的同一张图。
- 打开 export_tangents：apple 有法线贴图但图元无 TANGENT，缺失时切线由
  渲染器推导，在双面薄片叶子卡上不可靠。49/102 图元带上切线，顺带修正
  建筑法线贴图，GLB +0.83MB。

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

blender/export_cesium.py

@@ -29,6 +29,12 @@ from osmassets.materials import link_alpha_clip  # noqa: E402
 # mesh's shared slots can recognise its own output and leave it alone.
 EXPORT_PREFIX = "Cesium "
 
+# Fraction of its own albedo a cut-out foliage material emits, to keep the
+# shadowed side of a crown off Cesium's near-black ambient floor. Kept well
+# under the 0.18 the buildings use: a tree still has to read as lit from one
+# side, it just must not go to black.
+FOLIAGE_EMISSION = 0.22
+
 EXPORT_TINTS = {
     "Grass": ((0.12, 0.48, 0.08), 0.72),
     "Tree Crown Dark": ((0.06, 0.22, 0.05), 0.18),
@@ -362,6 +368,25 @@ def make_export_material(material):
         # SpeedTree atlas are black, so Cesium draws black slabs.
         link_alpha_clip(result, diffuse_node.outputs["Alpha"], bsdf,
                         cutoff=material.alpha_threshold)
+        # Lift the crown out of Cesium's ambient. The preview configures no
+        # environment map, so anything the sun does not hit directly falls to a
+        # weak default spherical-harmonic term — which is why every other
+        # material here carries an EXPORT_EMISSION_OVERRIDES entry. A crown is
+        # mostly self-shadowed leaf cards facing away from the sun, so at
+        # distance it collapses into one dark mass while a sunlit close-up
+        # still reads fine.
+        #
+        # Feed the diffuse back in as the emissive texture rather than using a
+        # flat colour: a constant would wash the bark with leaf green, whereas
+        # this floors every texel at a fraction of its own albedo. It costs no
+        # extra bytes — the exporter points emissiveTexture at the image the
+        # base colour already uses.
+        if "Emission Color" in bsdf.inputs:
+            links.new(diffuse_node.outputs["Color"], bsdf.inputs["Emission Color"])
+        elif "Emission" in bsdf.inputs:
+            links.new(diffuse_node.outputs["Color"], bsdf.inputs["Emission"])
+        if "Emission Strength" in bsdf.inputs:
+            bsdf.inputs["Emission Strength"].default_value = FOLIAGE_EMISSION
     elif "Alpha" in bsdf.inputs:
         bsdf.inputs["Alpha"].default_value = 1.0
     return result
@@ -452,6 +477,11 @@ def export(args):
         export_normals=True,
         export_materials="EXPORT",
         export_image_format="AUTO",
+        # The foliage materials carry a normal map, and glTF leaves tangent
+        # derivation to the renderer when TANGENT is absent. On thin
+        # double-sided leaf cards that derivation is unreliable, so ship real
+        # tangents — it costs four floats a vertex on meshes this small.
+        export_tangents=True,
         export_extras=True,
         export_cameras=False,
         export_lights=False,