Build Zeliard Creator content browser

[nolanvenhola]

Summary

• add the Zeliard Creator dock with searchable, kind-filtered content discovery
• create graph-valid templates for all eleven content types
• replace typed stable-ID references with kind-aware Inspector pickers
• add navigable validation results, dirty-state tracking, Save All integration, and specialized-editor hooks
• document the Creator workflow and editor-safe content contract

Validation

• architecture and typed GDScript checks
• editor import and Creator plugin startup
• production content graph validation
• 18 headless production/Creator/save scenarios
• main-scene smoke test
• Windows and Web release exports
• exported Windows runtime smoke

Closes #242