Add a render.json schema and editor autocomplete

[Swarek]

Goal

Add editor autocomplete and strict validation for fighters/<id>/render.json.

Acceptance criteria

• Add a versioned fighters/render.schema.json covering the 2D and 3D manifest variants.
• Cover every supported 2D action source type and its required fields.
• Generated render.json files include a $schema pointer.
• Unknown keys and misspelled fields are rejected where safe.
• Add schema fixtures or tests for one valid and one invalid 2D and 3D manifest.
• Link the schema from the fighter-authoring guide.

This should improve the authoring experience without changing existing ready packs unexpectedly.