loadEntropy

#252: load the wallet's 32-byte BIP-39 entropy (biometric-gated) — the source for a recovery phrase. Serialized on bootstrapMutex so it can't race a concurrent ensureSeedReady. Returns a fresh copy the caller MUST wipe; throws if no wallet exists.

This is the low-level seam behind WalletRecovery.revealPhrase — which is the recommended, phrase-shaped API. It's biometric-gated regardless (decrypting the vault prompts), so it's not a silent leak; prefer WalletRecovery unless you specifically need raw entropy.