Pattern: Claim Check

Deployable demo that illustrates large message handling with a claim check pattern.
- demonstrate how to read message contents in order to promote properties
- demonstrate how to store the large message
- self contained demo
- independently deployable demo
- independent documentation

The issue names no files, tests, or entry points. Start by reviewing the repository's existing deployable demos and documentation conventions, then scope a self-contained, independently deployable Claim Check demo covering property promotion, large-message storage, and its documentation; done means all listed demonstrations are included.

Written by the indexing model from the issue text.