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Title: Feature Request: Add Triangle Node Type to G6 v5

Title: Feature Request: Add Triangle Node Type to G6 v5: a task in LegoFlow-SWE (Harbor dataset). Description I’m building a graph visualization with G6 v5 and need to render nodes as triangles. The existing built-in node types cover Circle, Rect, and Star, but there is no Triangle node type yet.

The task

**Description** I’m building a graph visualization with G6 v5 and need to render nodes as triangles. The existing built-in node types cover Circle, Rect, and Star, but there is no `Triangle` node type yet.

Part of Lego-X/LegoFlow-SWE.