Function verify_signed_mldsa 

pub fn verify_signed_mldsa(
    env: &SignedRoomMessage,
    pinned_mldsa_pubkey: &[u8],
) -> Result<bool>

huddle 2.0.6 (WS2-a): verify an envelope’s composite ML-DSA-65 signature against a pinned ML-DSA public key (the caller’s durable record of this signer’s PQ-auth key, learned from a prior signed announce). The Ed25519 layer is checked separately by verify_signed; this is the additional post-quantum check, gated on having pinned the signer’s key.

• Ok(true) — a valid ML-DSA signature by the pinned key (PQ-auth confirmed).
• Ok(false) — no ML-DSA signature present (a classical-only envelope).
• Err(..) — the envelope claims a different ML-DSA key than pinned, or carries a malformed/invalid ML-DSA signature: a downgrade/forgery the caller MUST reject. (A caller that has pinned this signer should also treat Ok(false) — a stripped signature — as a downgrade and reject it.)