fix(tripo-3d): replace icon with solid black full-square artwork

[sunshakestrong]

Plugin type (required) / 插件类型（必填）

• UI Plugin / 是 UI 插件 — provides MCP Apps interactive pages / 提供 MCP Apps 交互页面
• Standard plugin / 普通插件（非 UI） — does not provide MCP Apps interactive pages / 不提供 MCP Apps 交互页面
• Not applicable / 不适用 — repository-only changes; no plugin content changed / 仅仓库级改动，不涉及插件内容

Affected plugin name(s) / 涉及的插件名称：tripo-3d

What / 改动内容

After ff03508 fix(assets): use solid full-square official plugin icons, the transparent rounded corners of the Tripo 3D icon were filled with a light lavender color (#F1ECFF). In dark theme these corners show up as white edges around the icon on the marketplace page.

This PR replaces assets/tripo-3d/icon.png with official artwork that is a solid black 512×512 full square (no baked corner radius, no transparency), matching the asset convention. The sha256 entry for tripo-3d in assets/icon-sources.json is updated accordingly.

ff03508 之后，Tripo 3D 图标原本透明的圆角被填成了浅紫色（#F1ECFF），在暗色主题的插件市场页面上表现为图标四角的白边。本 PR 将 assets/tripo-3d/icon.png 替换为满版纯黑底的 512×512 官方图（无内置圆角、无透明区域），符合资源目录约定，并同步更新 assets/icon-sources.json 中 tripo-3d 的 sha256。

Only the icon asset changes; the plugin package and version are untouched. / 仅改图标资源，不涉及插件包与版本号。

Self-check / 自查清单

• Plugin type and affected names are declared above / 已在上方声明插件类型与涉及的插件名称
• No secrets, machine-specific paths, or private dependencies / 不含密钥、机器专属路径或私有依赖
• python3 scripts/validate.py passes (OK: 43 plugin(s) validated) and python3 -m unittest tests.test_icon_assets passes (3 tests) / 本地校验与图标测试均通过

[panda920]

✅ Internal validation passed. A ZCode maintainer will review the change next; after acceptance, release checks run separately and this PR is closed automatically.