feat: deterministic grinding + random private key helper

[Jevaites]

Describe your changes

Adds a small, self-contained key generation utility that mirrors the key grinding logic used by other StarkNet SDKs, plus a convenience for generating a random private key.

Implementation

• StarkKeygen.grindKey(seed:maxIters:)
  Deterministically grinds an arbitrary seed into a valid Stark curve private key:
  • For i = 0..N: compute SHA256(seed || i)
  • Accept x only if x < limit, where limit = 2^256 - (2^256 mod n)
  • Return x % n (in [0, n)), where n is the Stark curve subgroup order
• StarkKeygen.randomPrivateKeyHex()
  Pulls 32 bytes from a CSPRNG and returns a 0x-prefixed, 64-hex-chars private key, produced by grindKey.

Both functions are pure and do not touch global state.

Public API

public enum StarkKeygen {
    public static func grindKey(seed: Data, maxIters: Int = 100_000) throws -> BigUInt
    public static func randomPrivateKeyHex() throws -> String
}

Errors

• StarkKeygenError.rngFailure – CSPRNG failure (only for randomPrivateKeyHex)
• StarkKeygenError.grindExceeded – exceeded maxIters without finding a value < limit

Breaking changes

• This issue contains breaking changes