`@remotion/drag-and-drop`: Stabilize the Element install contract

[samohovets]

Goal

Stabilize and document the contract used by Element libraries to request installation into Remotion Studio.

The current official library uses installInStudio() from @remotion/drag-and-drop. Before the Elements v1 release, define which parts form the supported protocol and how the design can later expand to other library authors.

Tasks

• Document the request payload, protocol version, compatibility requirements, and expected responses.
• Define the supported installInStudio() behavior for the official Elements library.
• Review naming and package ownership, including whether the protocol should remain in @remotion/drag-and-drop.
• Document the current origin policy and decide how third-party library support should evolve.
• Cover target selection, unsupported Studios, rejected requests, and malformed payloads with tests.
• Test the complete flow in a clean supported Remotion project.

Acceptance criteria

• The v1 installation contract is explicit and documented.
• The official gallery does not depend on undocumented Studio internals.
• Future third-party support has a deliberate extension path without requiring it to ship in v1.

Part of the Remotion Elements v1 release, #9769.