docs: add Passkey signing integration guide (#78)

[Peolite001]

Closes #78

Description

This PR implements the requested documentation for integrating a Passkey (or any smart-account) signer into stealth derivation, resolving issue #78.

Changes Included:

• New Guide: Added guides/stellar/passkey-signing.mdx.
• StellarStealthSigner Interface: Fully documented the interface required for integrating custom signers.
• Reference Adapter: Included a functional TypeScript snippet demonstrating how to implement a Passkey signer adapter using the WebAuthn API.
• Trust Boundary Analysis: Added an explicit section outlining what a Passkey secures (private key material within the device's secure enclave) and what it does not (the integrity/validation of the transaction contents).
• Determinism Note: Documented the non-deterministic nature of standard Passkeys (ECDSA P-256) and how it affects strict deterministic requirements in stealth key derivation protocols.

Related Issues

• Resolves Passkey signing integration guide #78
• Paired with sdk issue Stellar stealth cryptography deep-dive page #3

Acceptance Criteria Met

• Snippets pass CI / are valid TypeScript.
• Interface fully documented.
• Trust boundary made explicit.

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[truthixify]

Merged. Passkey guide pairs nicely with the SDK signer interface that just landed.