[Security]: X3DHKeyManager.verifyBundle doesn't verify signed-prekey signature

[imran-siddique]

In \packages/agent-mesh/sdks/typescript/src/encryption/x3dh.ts, \�erifyBundle()\ only checks field lengths — it does NOT perform Ed25519 signature verification on the signed prekey.

Additionally, \PreKeyBundle\ schema doesn't include the Ed25519 identity key needed for verification (only has X25519 key).

Fix needed:

1. Add \identityKeyEd\ (Ed25519 public key) to PreKeyBundle schema
2. Call \�d25519.verify(sig, signedPreKey, identityKeyEd)\ in verifyBundle()

This is a real crypto-binding regression vs the vendored SDK.

Ref: Azure/kars#27 review (blocker #6)

[lawcontinue]

The missing signature verification means a MITM can swap the signed prekey without detection — the bundle looks valid but the binding between identity and ephemeral key is unverified.

We hit a similar gap when implementing Ed25519 agent identity for a distributed pipeline: the handshake verified key possession but not key binding. The fix was the same pattern — include the identity public key in the bundle and verify the signature chain (identity → signed prekey → one-time prekey). Worth also adding a bundle hash commitment so the receiver can detect tampering on any field, not just the signed prekey.