Add BCR-2026-003: XID Edges for verifiable claims between entities

Add new research paper defining edges as the mechanism by which XID
documents represent verifiable claims. Edges connect a source (claimant)
to a target entity, typed by ontological concepts, with independent
signature verification. Add paper entry to the README index.

Author: wolfmcnally

README.md

@@ -88,6 +88,7 @@ Each BCR has a status which is indicated by a symbol.
 | [BCR-2025-006](papers/bcr-2025-006-hubert.md) | Hubert: Distributed Key-Value Store for Secure Multiparty Coordination | Wolf McNally | ⭐️ |
 | [BCR-2026-001](papers/bcr-2026-001-unit.md) | Unit: The Known Value for Deliberate Emptiness | Wolf McNally | ⭐️ |
 | [BCR-2026-002](papers/bcr-2026-002-envelope-notation.md) | Gordian Envelope Notation - Quick Reference | Wolf McNally | ⭐️ |
+| [BCR-2026-003](papers/bcr-2026-003-xid-edges.md) | XID Edges: Verifiable Claims Between Entities | Wolf McNally | 📙 |
 
 _Also see our [Testimony](https://github.com/BlockchainCommons/Testimony/blob/master/README.md) and our [Blockchain Commons Proposals](https://github.com/BlockchainCommons/bcps/blob/master/README.md)._