[Index / structure] bridgelet-product-audit/integration-notes/README.md: Integration-notes subfolder index

[phertyameen]

Index / structure: bridgelet-product-audit/integration-notes/README.md

Purpose:
Create an index file listing every integration note with a one-line scope summary and link.

This file should cover:

• Group entries by which layer they primarily concern (wallet/signing, claim flow, analytics, mobile).
• Note that these notes specifically document points of contact between this repo's clients and the bridgelet-sdk/bridgelet-core layers beneath them.
• Keep the index itself free of new findings — it should only route readers to the right document.

Scope: documentation-only addition to a new, standalone folder. No changes to frontend/ or mobile/ are needed to close this issue.

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Part of the bridgelet-product-audit/ knowledge-base initiative (96/100).