Register nested sourced_from path segments on the root index

[ellisandrews-toast]

What

Populates IndexDefinition#sourced_from_nested_paths_by_relationship (stubbed to {} until now). When a parent_relationship chain resolves, its path segments are converted into ListPathSegment/ObjectPathSegments and registered on the root indexed type's Index, keyed by the leaf relationship name.

Scope

Path registration is the only effect of a resolved chain in this PR — no nested update target is produced yet (that comes later in the PR stack). Every parent_relationship chain is still resolved so its configuration errors surface, but a path is registered only for relationships that actually back a sourced_from field.

Validation

Index#register_sourced_from_nested_paths raises a clear SchemaError if two chains terminating at the same index share a relationship name — the runtime tracks sourced data by relationship name (__versions/__sources), so a collision can't be represented and we fail fast at schema-definition time rather than silently clobbering.

Notes

• These tests assert on the in-memory runtime-metadata objects (the schema-definition logic that produces the path segments). Serialization of those segments (to_dumpable_hash/from_hash) is already round-trip-covered in elasticgraph-schema_artifacts's schema_spec.rb, so it isn't re-tested here.
• No test schema exercises a nested chain yet, so there's no runtime_metadata.yaml churn. A later PR will add a nested sourced_from schema to config/schema/ and assert on the dumped YAML end-to-end.