Use CryptoKit's HKDF primitive for the Paykit receiver Noise key derivation

[jvsena42]

Follow-up from a review comment on #630: #630 (comment)

can be replaced with the primitive HKDF<SHA256>.deriveKey(inputKeyMaterial:salt:info:outputByteCount: 32)

Context

PaykitReceiverNoiseKeyDerivation.derive in Bitkit/Services/PubkyService.swift hand-rolls HKDF-SHA256:

static func derive(seed: Data, network: String, receiverPath: String) -> Data {
    let domainBytes = Data(domain.utf8)
    let salt = Data(SHA256.hash(data: domainBytes))
    var prk = Data(HMAC<SHA256>.authenticationCode(for: seed, using: SymmetricKey(data: salt)))
    defer { prk.resetBytes(in: prk.startIndex ..< prk.endIndex) }

    var expandInput = Data("\(version)\0\(network)\0\(receiverPath)".utf8)
    expandInput.append(0x01)
    return Data(HMAC<SHA256>.authenticationCode(for: expandInput, using: SymmetricKey(data: prk)))
}

This is exactly HKDF-Extract-and-Expand for a single 32-byte output block, so it is equivalent to:

HKDF<SHA256>.deriveKey(
    inputKeyMaterial: SymmetricKey(data: seed),
    salt: salt,
    info: Data("\(version)\0\(network)\0\(receiverPath)".utf8),
    outputByteCount: 32
)

Why

Removes hand-rolled crypto from the wallet in favour of the platform primitive. The output bytes are identical, so this is a pure refactor.

Acceptance

• PaykitReceiverNoiseKeyDerivation.derive uses HKDF<SHA256>.deriveKey.
• PaykitReceiverNoiseKeyStoreTests.testDerivationMatchesVersionedCrossPlatformVector still passes unchanged — it pins the cross-platform vector shared with Android (500f4799bbb2d02103e3b74b365ddb478a3187333c053fa9eb62f4052ba6a327), so a green test is the parity guarantee.
• No change to the derived key for existing users (a changed key would fail closed in PaykitReceiverNoiseKeyStore.validatedKeyBytes and break Paykit).