fix(free-layout): emit content changes for global variable edits

[Gujiassh]

Summary

Fixes #986.

Global variable edits in free-layout were changing variable state without triggering the preset onContentChange callback, so autosave and other content listeners could miss updates from the variable panel.

Changes

• bridge GlobalScope list/value updates into the preset onContentChange callback when the variable engine is enabled
• add a preset-level regression test that updates the global variable schema and asserts onContentChange fires

Checklist

• added focused regression coverage
• kept the change scoped to free-layout-editor only
• verified the package locally

Release Impact

Free-layout editors that enable the variable engine now receive onContentChange callbacks when global variables are edited from the variable panel.

Validation

• cd packages/client/free-layout-editor && npm test
• cd packages/client/free-layout-editor && npm run ts-check
• cd packages/client/free-layout-editor && npm run build
• git diff --check