docs: add Java payload offloader

[zhongkechen]

Summary

• replace the superseded Java SerDes-pipeline documentation with the dedicated PayloadOffloader design
• document FileSystemPayloadOffloader as independent from existing custom SerDes configuration
• add Java examples for global and per-operation offloading, storage modes, path encoding, and structured previews
• document the dedicated payload I/O executor, retry wrapper, immutable publication, external lifecycle ownership, capability handling, and preview immutability
• retain the existing idiomatic TypeScript and Python filesystem SerDes APIs

Related changes

Part of the Java payload-offloading stack:

• docs: record Java payload offloading architecture aws-durable-execution-sdk-java#678 — architecture decision
• feat: add payload offloader core runtime aws-durable-execution-sdk-java#649 — core runtime and API
• feat: add filesystem payload offloader and retries aws-durable-execution-sdk-java#681 — filesystem implementation and retries
• test: add payload offloader runners and integration coverage aws-durable-execution-sdk-java#682 — testing utilities and integration coverage
• docs: add payload offloader examples and E2E coverage aws-durable-execution-sdk-java#683 — examples and E2E coverage

Testing

• python3 scripts/check_example_refs.py
• git diff --check
• CI runs Markdown formatting, spelling, example-reference, and Zensical build checks

Resolved