Replace retrieval synonyms with glossary terms

## Parent

#406

## What to build

Replace unsupported alternate spellings of the package's core retrieval terms after the `parallel_chunks` explanation has been settled. Where "sub-request" denotes one piece of a split query, use chunk; where concurrency is described, use chunks in flight. Where "parallelism" denotes concurrent chunk execution, use fan-out.

Apply the correction across package prose, user documentation, examples, notebooks, comments, and wording-sensitive test messages. Preserve ordinary uses that are not synonyms, references in accepted decisions explaining rejected names, and the independent-item meaning of batch.

## Acceptance criteria

- [ ] "Sub-request" no longer denotes a chunk anywhere in tracked package, documentation, examples, notebooks, or tests.
- [ ] Concurrency descriptions consistently use chunks in flight.
- [ ] Uses of "parallelism" that denote concurrent chunk execution are rewritten as fan-out.
- [ ] Accepted-decision references to rejected names and ordinary behavioral uses remain unchanged.
- [ ] Batch terminology for independent-item skip policy remains unchanged.
- [ ] No public identifier, parameter, environment variable, signature, or runtime behavior changes.
- [ ] Repository-wide searches demonstrate that every remaining occurrence is an explicit exclusion rather than unresolved drift.
- [ ] Notebooks and documentation build successfully, and the offline suite, lint, type checker, and architecture checks pass.

## Blocked by

- #412