Data Binding

Requirements:
- Properties within a document should be bindable using a Mustache style syntax. (Double curly braces on each side)
- Data can be also be sourced from an external file. Perhaps this should be in a future version.

No files, tests, or entry points are named. Start by locating how documents and their properties are parsed and rendered, then determine how Mustache-style bindings and external data would fit that flow. Done should include a defined binding syntax and behavior, with the external-file source treated according to the project’s agreed scope.

Written by the indexing model from the issue text.