Add README and core design docs (architecture, roadmap, scene model, loop ownership, ML direction)

[dragosandreibobu]

Motivation

• Capture the project's intent and high-level direction for evolving the prototype into a coherent engine/editor stack.
• Provide a shared reference for engine/editor/runtime responsibilities, scene serialization, and a modernization path.
• Surface an initial ML-native direction so model work can be planned as first-class engine features.

Description

• Add README.md describing the project scope, documentation map, current architectural center, and recommended future identity.
• Add docs/architecture-vision.md to define the target dependency direction, core layers, and engine/editor/runtime responsibilities including example APIs.
• Add docs/scene-json-model.md and docs/runtime-loop-ownership.md to specify the scene/JSON model, runtime loading flow, and the rule that hosts own the main loop with example host call patterns.
• Add docs/ml-native-direction.md and docs/modernization-roadmap.md to propose a minimal C++ ML module, demo ideas, and a phased modernization roadmap with success conditions.

Testing

• No automated tests were added for this documentation-only change.
• No automated tests were run as part of this PR; CI linting or doc checks may validate these files when run in the project pipeline.

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Codex Task