devSeedBytes

Dev-only override: when isDebug is true and hex is a 128-char hex string (64 bytes), decode and return; otherwise return null and let the PRF path run.

Extracted so the gate is unit-testable without standing up a BuildConfig fixture. The call site passes BuildConfig.DEBUG + BuildConfig.DEV_SEED_HEX; tests pass whatever values they need.

The 64-byte length is enforced because anything else would silently break BIP-32 derivation downstream — fail loud here instead of producing a wallet at the wrong address.