Cryptographically secure numeric one-time-password primitive for the mix skua.gen.auth OTP flows.

The generated auth code calls into this so the security-critical bits live in one tested place rather than copy-pasted (and possibly mangled) into every app:

• generate/1 — an unbiased N-digit numeric code via CSPRNG rejection sampling, returned as a fixed-width string with leading zeros preserved.
• hash/1 — SHA-256 of the code string. Store the hash, never the code.
• normalize/2 — coerce a submitted code to a canonical digits-only string and validate its length. Pins the wire format: rejects short/long or integer-coerced input rather than zero-padding it (which would otherwise collapse the code space or silently fail leading-zero codes).
• valid?/3 — constant-time comparison of a submitted code against a stored hash, after normalization.

Security properties (each is covered by a test)

• No modulo bias. Codes are drawn with :crypto.strong_rand_bytes/1 and rejection sampling, so every value in 0..(10^digits - 1) is equally likely.
• Leading zeros are significant. A 6-digit code is always a 6-character string; "004217" hashes and compares as "004217", never 4217.
• Constant-time compare. Verification uses Plug.Crypto.secure_compare/2 to avoid timing leaks.
• Configurable length. 6 digits by default; 8 is recommended for high-value deployments (generate(8)), with the same guarantees.