Define save/load semantics for the invented scene root entity

[apotema]

Part of #560 (RFC: unify scenes and prefabs).

Problem

Scenes today have no root entity; unification invents one. Existing save files were written under the no-root model — so the synthetic root is a potential save-format break. The RFC's "Out of scope" covers hot-reload but not save compat.

Decision to make

Whether the synthetic root is persisted:

• Persisted + tagged engine-synthetic (recommended) — a marker component lets tools/diffs filter it; destroy-root has a stable, save-visible handle.
• Not persisted — needs a stable hidden handle reconstructed on load; fragile across reload (the "remembered one, forgot the other" failure mode the RFC exists to kill).

Scope

• Pick and specify the approach.
• If saves exist for in-tree projects, define the load-path behavior for old (rootless) save files: either a migration on load, or graceful synthesis.
• Normative spec lands in the RFC doc.

Acceptance

• An old (pre-unification) save file loads correctly post-unification.
• Round-trip save -> load -> save is stable (no spurious root churn in diffs).

Refs: #560, #470.