feat: add paykit module

[coreyphillips]

This PR:

• Adds the paykit module and core functions
• Adds docs & tests as needed
• Bumps version to 0.1.32
• Update bindings accordingly

New Methods

Session Functions

• createPubkyRingSessionRequest(callbackUrl, additionalParams?) - Generate URI to request session from Pubky Ring
• parsePaykitDeeplink(url) - Parse a Paykit deeplink URL
• validatePaykitDeeplink(url) - Validate a Paykit deeplink URL
• createDeeplinkFromToken(baseUrl, action, token, additionalParams?) - Create deeplink from session token
• createSessionTokenFromKeypair(publicKey, secretKey, homeserverUrl?, expiresInSeconds?) - Create session token from keypair
• serializeSessionToToken(sessionData) - Serialize session data to token
• deserializeTokenToSession(token) - Deserialize token to session data

Standalone Functions

• setPaymentEndpoint(client, method, data) - Store/update a payment endpoint
• removePaymentEndpoint(client, method) - Remove a payment endpoint
• getPaymentList(reader, payee) - Get all payment methods for a user
• getPaymentEndpoint(reader, payee, method) - Retrieve a specific payment endpoint
• getKnownContacts(reader, key) - Get user's known contacts

PubkyAuthenticatedTransport

• PubkyAuthenticatedTransport() - Constructor (returns error, use token-based creation)
• setPaymentEndpoint(method, data) - Store/update a payment endpoint
• removePaymentEndpoint(method) - Remove a payment endpoint

PubkyUnauthenticatedTransport

• PubkyUnauthenticatedTransport() - Constructor for read operations
• getPaymentList(payee) - Get all payment methods for a user
• getPaymentEndpoint(payee, method) - Retrieve a specific payment endpoint
• getKnownContacts(key) - Get user's known contacts

Scanner

• New Scanner.PaykitSession variant for scanning Paykit session QR codes

New Types

• SessionToken - Base64-encoded session token wrapper (token: String)
• SessionData - Serializable session data (publicKey, secretKey, homeserverUrl?, expiresAt?, metadata?)
• MethodId - Payment method identifier (id: String)
• EndpointData - Payment endpoint data payload (data: String)
• PublicKey - Pubky public key wrapper (key: String)
• SupportedPayments - Map of method IDs to endpoint data (entries: Map<String, EndpointData>)
• PaykitDeeplink - Parsed deeplink (action, sessionToken?, parameters)
• ScannedPaykitSession - Scanned session from QR code (url, action, token, parameters)
• PaykitException / PaykitError - Error types

Documentation

See src/modules/paykit/README.md for Swift and Kotlin implementation examples.