chore: add changeset for signature support

Author: satyam-mishra-pce

.changeset/fuzzy-pandas-smile.md

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+---
+"@hypercerts-org/lexicon": minor
+---
+
+Add cryptographic signature support to all lexicons
+
+Adds optional cryptographic signature support to all record lexicons, enabling platform attestation and verification that records were created through trusted services. This is inspired by the [ATProtocol Attestation Specification](https://ngerakines.leaflet.pub/3m3idxul5hc2r) by Nick Gerakines.
+
+**New lexicons:**
+
+- `app.certified.signature.inline` - Inline cryptographic signature object with JOSE algorithm identifiers (ES256, ES256K, Ed25519)
+- `app.certified.signature.list` - Container object with `signatures` array supporting both inline signatures and remote attestation references
+- `app.certified.signature.proof` - Remote attestation proof record containing the CID of attested content
+
+**Changes to existing lexicons:**
+
+All 19 record lexicons now include an optional `signatureData` property that references `app.certified.signature.list`:
+
+- `org.hypercerts.claim.activity`
+- `org.hypercerts.claim.contribution`
+- `org.hypercerts.claim.contributorInformation`
+- `org.hypercerts.claim.rights`
+- `org.hypercerts.collection`
+- `org.hypercerts.context.acknowledgement`
+- `org.hypercerts.context.attachment`
+- `org.hypercerts.context.evaluation`
+- `org.hypercerts.context.measurement`
+- `org.hypercerts.funding.receipt`
+- `org.hypercerts.workscope.tag`
+- `org.hyperboards.board`
+- `org.hyperboards.displayProfile`
+- `app.certified.badge.award`
+- `app.certified.badge.definition`
+- `app.certified.badge.response`
+- `app.certified.actor.profile`
+- `app.certified.actor.organization`
+- `app.certified.location`
+
+**Design notes:**
+
+This is a non-breaking extension - signatures are optional on all records. Two signature patterns are supported:
+
+1. **Inline signatures** - Embedded directly in the record via `app.certified.signature.inline`
+2. **Remote attestations** - References to proof records in other repositories via `com.atproto.repo.strongRef`
+
+Signature algorithm identifiers use [JOSE format](https://www.iana.org/assignments/jose/jose.xhtml) per ATProto convention:
+
+- `ES256K` - secp256k1 (Ethereum/Bitcoin curve, ATProto default)
+- `ES256` - P-256 (NIST curve, WebCrypto compatible)
+- `Ed25519` - EdDSA