Add public sign() method to Sphere for message signing

[jvsteiner]

Problem

There is no public API to sign arbitrary data using the wallet's identity key. The sphere.identity getter deliberately strips privateKey from the returned object, and the underlying _identity field is TypeScript-private.

Applications that need to prove identity (e.g. authenticating HTTP requests to a service) are forced to bypass access control:

const fullIdentity = (sphere as any)._identity;
const privateKey = fullIdentity.privateKey;

They must then bring in @noble/curves/secp256k1 and @noble/hashes as separate dependencies and reimplement signing from scratch — even though the SDK already depends on these libraries internally.

Context

We built two marketplace applications on top of sphere-sdk (a prediction market and a P2P marketplace). Both needed to authenticate HTTP requests using the wallet's secp256k1 keypair. Both independently implemented identical signing code (~30 lines each) that:

1. Accesses (sphere as any)._identity.privateKey
2. Hashes a payload with SHA256 (from @noble/hashes)
3. Signs with secp256k1 (from @noble/curves)
4. Returns the signature + compressed public key

The SDK has SigningService internally for on-chain state transitions, but it's not exposed for general-purpose signing. The SDK also already exposes deriveAddress() which returns private keys, and exportToJSON() which dumps the master key — so there's no philosophical objection to key access, just a missing convenience method.

Proposed Solution

Add a public method to Sphere:

sphere.sign(message: Uint8Array): { signature: Uint8Array, publicKey: Uint8Array }

Or with hex convenience:

sphere.signMessage(payload: string): { signature: string, publicKey: string }

This would:

• Eliminate the (sphere as any)._identity hack in every consumer
• Remove the need for consumers to directly depend on @noble/curves and @noble/hashes
• Provide a safe, supported way to prove wallet ownership without exposing raw private keys

Alternatives

• Expose sphere.getPrivateKey(): string — simpler but less safe, pushes signing responsibility to consumers
• Document the _identity access pattern — legitimizes the hack but doesn't solve the dependency duplication

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