deps: @antv/x6 major upgrade v2 -> v3

[ericfitz]

Summary

@antv/x6 has a major version update available: 2.19.2 -> 3.1.6.

Currently pinned to exact version 2.19.2 in package.json with comment: "Pinned to v2.x - v3.x has breaking changes that require significant refactoring."

Impact

• No security implications — this is a feature/breaking change upgrade
• v3.x has breaking API changes affecting the DFD (Data Flow Diagram) module
• All @antv/x6-plugin-* packages may also need coordinated updates

Action Required

• Plan migration on a feature branch when DFD refactoring is scheduled
• Review v3.x migration guide and breaking changes
• Update @antv/x6 and all related plugins together
• Test DFD diagram creation, editing, and rendering thoroughly

[ericfitz]

Update information (Chinese):
https://x6.antv.antgroup.com/tutorial/update

Translated to English:
https://x6-antv-antgroup-com.translate.goog/tutorial/update?_x_tr_sl=zh-CN&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp

[ericfitz]

Blocked by bug in declaring ESM vs CJS: antvis/X6#5048