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

Commencez par lire l’issue parente #406 et l’issue bloquante #412 afin de comprendre les termes du glossaire établis. Recherchez les synonymes indiqués dans les textes des paquets suivis par Git, la documentation, les exemples, les notebooks, les commentaires et les tests sensibles à la formulation, en préservant les exclusions explicites. C’est terminé lorsque les recherches dans tout le dépôt ne montrent plus que des exclusions intentionnelles, que la documentation et les notebooks se construisent, et que la suite hors ligne, le lint, le vérificateur de types et les contrôles d’architecture passent.

Rédigé par le modèle d'indexation à partir du texte de l'issue.