Add iter()/aiter() for filter-based lazy iteration #489

Summary

Add lazy filter-based ID/key iteration APIs for index maintenance and agent pipelines.

Level: Intermediate

Current State

• paginate() yields full document records, not IDs/keys only.
• scan_by_pattern() works on raw Redis key patterns and returns a materialized list.
• No index filter-expression based generator for IDs/keys.

Proposed Change

• Add lazy iterators over keys included in an index in Redis optionally applying some filtering:
  • iter(filter_expression...)
  • aiter(filter_expression,...)
• Yield as a generator of said keys?

Definition of Done

• Iteration is lazy (streaming batches, no full list required).
• Works for selective and wildcard filter expressions.
• Integration tests verify complete and deterministic coverage.

Out of Scope

• Arbitrary cursor checkpoints or resumable iterators.