Speed up editor domain reloads

[marek-jelo-jelinek]

Reduces domain reload time by removing two expensive operations from DashEditorCore static constructor:

• Lazy-load graph assets: GraphAssets no longer scans and loads every graph asset on each domain reload. The collection is now populated on first access via the GraphAssets getter.
• Optimize execution-order setup: SetExecutionOrder now loads the required script directly from its known path using AssetDatabase.LoadAssetAtPath, instead of using Resources.FindObjectsOfTypeAll, which loads every MonoScript in the project.

There is no behavior change as this only avoids unnecessary work during domain reloads.