feat(stellar): add signer interface for passkey stealth key derivation

[collinsezedike]

Summary

• Add StellarStealthSigner interface (signMessage(message): Promise<Uint8Array>) and deriveStealthKeysFromSigner() in src/chains/stellar/signer.ts / keys.ts, so stealth key derivation no longer assumes a synchronous Freighter-shaped ed25519 signature.
• FreighterStealthSigner wraps the existing Freighter-style wallet API; the raw deriveStealthKeys(signature) path is unchanged.
• WebAuthnPasskeyStealthSigner is a reference passkey adapter. WebAuthn get() assertions aren't deterministic across sessions (fresh ECDSA nonce, changing counter/clientDataJSON), so instead of hashing the assertion signature it uses the WebAuthn Level 3 prf extension, which is deterministic for a given credential and salt.
• useStellarStealthKeys() in @wraith-protocol/sdk-react gained generateFromSigner() alongside the existing generate().
• Docs in docs/chains/stellar-stealth-signers.md, changelog entry under the upcoming release.

Closes #121.

Semver

Additive-only: new exports (StellarStealthSigner, deriveStealthKeysFromSigner, FreighterStealthSigner, WebAuthnPasskeyStealthSigner) plus a new hook method. No existing signature or behavior changes.

Test plan

• pnpm build (root and sdk-react)
• pnpm test — full Stellar suite (797 passed) and sdk-react suite pass
• New test/chains/stellar/signer.test.ts: Freighter path unchanged, passkey mock derives identical keys across two independent sessions, different credentials diverge, missing PRF / missing credentials container throw
• pnpm format:check

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