docs: align Realtime architecture references

[ikenyal]

Problem

Several public-facing descriptions still identify Realtime as AppSync Events or IoT even though the current implementation provisions API Gateway WebSocket with DynamoDB-backed connection tracking. This affects architecture, service-limit, and cost decisions for users of the bb-realtime package and the blocks umbrella package, as well as contributors using the protocol-extension references.

Issue #, if available: N/A

Changes

• Correct the Realtime public JSDoc and umbrella-package description to name API Gateway WebSocket + DynamoDB.
• Replace obsolete AppSync-specific scaling and cost claims with accurate descriptions and a link to the Realtime documentation.
• Align Agent design/cost documentation, the Agent source comment, and the comprehensive-app FAQ with the deployed architecture.
• Update the protocol-extension examples and Transferable design reference to show API Gateway Management API fan-out and DynamoDB-backed connection tracking instead of AppSync/IoT.
• Keep AppSync references that explicitly describe the previous implementation or list it as an external alternative.

Validation

• Ran npm run sync-docs:check.
• Ran git diff --check.
• Verified the revised descriptions against the Realtime CDK and AWS runtime implementations, README, and design document.
• Confirmed that remaining AppSync references in the affected areas are explicitly historical or describe an external alternative.

No automated tests were added because this is a documentation-only correction. No changeset is needed because no published package behavior changes.

Checklist

• PR description included
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[changeset-bot]

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