Add the modern pixel-art import pipeline

[nolanvenhola]

Summary

• add deterministic Aseprite-to-PNG export tooling and contract coverage
• enforce lossless project-wide PNG import defaults with Creator hot reload
• store pivots, frame grids, clips, palette limits, and provenance in stable typed resources
• add actionable art diagnostics and exact-pixel/metadata-preservation tests
• document the production art workflow

Validation

• architecture and typed GDScript checks
• Aseprite CLI export contract (deterministic stand-in)
• editor import and Creator plugin startup
• production content and pixel-art import validation
• 22 headless production/Creator/art/save scenarios
• main-scene smoke
• Windows and Web release exports
• exported Windows runtime smoke

Closes #244