feat: add optional plugin dependency support

[Yonava]

Summary

Introduces optionalDependsOn to the GraphPlugin type, allowing plugins to declare soft dependencies that may or may not be present at runtime.

• Adds optionalDependsOn field to LoosePluginData and DefaultPluginData, with controls typed as Partial and events merged into the hub without ordering enforcement
• Converts anchors' hard dependency on focus to an optional one, replacing three TODO-guarded controls.focus.* calls with controls.focus?. null-safe access

This unblocks plugin compositions where anchors is used without focus, removing the need for the workarounds previously tracked via TODO comments.