Module @blockchaincommons/rand

@blockchaincommons/rand - random number primitives for the Blockchain Commons stack.

• RandomNumberGenerator: the contract (nextU32, nextU64, fillBytes).
• SecureRng / secureRng: Web Crypto backed.
• SeededRng: deterministic xoshiro256**, identical to the Rust and Swift implementations for the same seed; SeededRng.forTesting() is the shared cross-platform fixture seed; state and clone() persist and fork it.
• randomBytes, fillRandomBytes, randomBool: helpers taking { rng } and defaulting to the secure generator.
• RandError: the one error type, with a code for every failure.
• Bounded and ranged integer samplers live in @blockchaincommons/rand/samplers.