Reference external proto enum types

[jwils]

Why

Some schemas already have canonical protobuf enum definitions, and generating duplicate enums in schema.proto makes Java consumers deal with parallel types.

What

• Add proto_external_types to import and reference existing proto enum types
• Validate external enum references against exactly one option-free proto_enum_mappings source
• Preserve generated local definitions for transformed, curated, multi-source, or non-enum types
• Document the enum-only API and add focused generator/error coverage

Risk Assessment

Low — this only affects the unreleased elasticgraph-protobuf extension from #1080, and external references are opt-in with validation.

References

• Stacked on Add elasticgraph-protobuf schema artifacts #1080