Make _RecordPreparer generic

[myronmarston]

Why

A record preparer only handles records of the ingestion format it serves. The interface declared a method-level generic ([T < ::Object]), so any caller could pass any record type and get the same type back. That signature cannot express the relation between a preparer and its format, and it gives no protection when a format with a non-Hash record type is added.

What

• _RecordPreparer[R], where prepare_for_index takes and returns R.
• RecordPreparer, RecordPreparer::Identity, and each caller use _RecordPreparer[untyped] until a concrete record type exists.

RBS only. No runtime change.

🤖 Generated with Claude Code

[myronmarston]

This change is part of the following stack:

• Point test datastore URLs at 127.0.0.1 instead of localhost #1400
  • Move record id preparation into the JSON ingestion adapter #1393
    • Report indexing failures through a _IndexingFailure interface #1395
      • Make _RecordPreparer generic #1396 ◀
        • Validate the event envelope before building operations #1397
          • Route events through a typed Event value #1398

_{Change managed by git-spice.}