Value provenance tracking

To improve debugging, it would be beneficial to track the provenance of values (where values come from), and show this information in error messages, or through specialized evaluation modes.

Some ideas:

* Capture both source sections and stack frames when values are initialized
* Map source sections in rendered output (e.g. YAML) back to provenance information

Start by tracing how values are initialized, how source sections and stack frames are represented, and how rendered YAML output is produced. Define the provenance information needed for error messages and specialized evaluation modes, then verify that source sections in rendered output map back to their originating values.

Written by the indexing model from the issue text.