Support sourcing list fields onto a nested element (with tracked __counts)

## Background

ElasticGraph's `sourced_from` feature lets a field on an indexed type be populated from a *different* source type's events, rather than from the indexed type's own events. https://github.com/block/elasticgraph/pull/1252 extended this so a `sourced_from` field can live on a **nested element** of an indexed document, not just at the top level of the document.

Separately, ElasticGraph maintains a hidden `__counts` map on indexed documents to track the sizes of list fields. When a list field's contents change, ElasticGraph recomputes its count so that list-size-based queries stay correct.

## Feature gap

Nested `sourced_from` currently supports sourcing **scalar** and **object** fields onto a nested element, but not **list** fields. In order to support sourcing nested **list** fields, the `__counts` tracking must work for these fields (there is currently no mechanism for this for the nested sourced field case).

## Interim state

Until this is supported, a schema-dump-time validation will reject defining such a schema with a clear error pointing to this issue.

Start by tracing the existing nested `sourced_from` handling and the hidden `__counts` tracking for list fields. Review the schema-dump-time validation that currently rejects this case; done means nested sourced list fields are supported with correct counts and are no longer rejected by that validation.

Written by the indexing model from the issue text.