Explicit Engine/client form title API

The engine does not currently expose a clear way to access the title of a form. We will likely want to build on existing mechanisms to expose this, namely:

* The `TextRange` abstraction already used for conveying other aspects of text
* Exposed as a getter on the primary instance top level node’s `currentState` object

While we don’t presently support the dynamic aspects of these interfaces, reusing the common pattern will add helpful consistency now… and leave the door open for functionality we can anticipate adding in the future (such as potentially applying form translations to the title itself; or some limited amount of text formatting).

Start by tracing the existing TextRange abstraction and the primary instance top-level node’s currentState object, since the issue identifies these as the intended integration points. Confirm how other text properties are exposed and define completion as making the form title available through a consistent getter while preserving the existing interface pattern.

Written by the indexing model from the issue text.