Prefab system (reusable object templates with overrides)

Current State

GameObjects can only be authored inline per scene. There is no way to define a reusable template (Unity prefab / Godot scene-instance) and stamp many instances that stay linked to the source.

Goal

A Prefab asset: a serialized GameObject subtree that can be instantiated into scenes, with per-instance overrides and propagation of source edits.

Tasks

• Prefab asset format (a GameObject subtree) with GUID/.meta, built on Scene serialization (JSON)
• Create-prefab-from-selection in the Scene Tree
• Instantiate into a scene (runtime + editor); instance stores a link to the source prefab GUID
• Per-instance overrides (changed fields) with visual indication in the Inspector
• Apply/revert overrides; propagate source-prefab edits to instances
• Nested prefabs
• Drag prefab from Asset Browser into viewport to instantiate (uses #24)

Acceptance Criteria

• A prefab can be created from a scene object and instantiated multiple times.
• Editing the source prefab updates instances except where overridden.
• Overrides are visible and can be reverted.

Related

• (serialization), (scene tree), #24 (drag to instantiate), #11 (closed) (asset identity).