Function prepare_shadow 

pub fn prepare_shadow(
    all_y_positions_sorted: &[CoeffPos],
    shadow_pass: &str,
    message: &str,
    files: &[FileEntry],
    parity_len: usize,
) -> Result<ShadowState, StegoError>

Prepare a shadow layer for embedding.

Builds the payload, encrypts, frames, RS-encodes, then selects positions using cost-pool filtering (Tier 1) + hash permutation (Tier 2). Tries fractions from smallest (best stealth) to largest (most capacity).

all_y_positions_sorted must be sorted by cost (cheapest first).