Function pack 

pub fn pack(
    source_dir: &Path,
    manifest: &Presentation,
    output: &Path,
) -> Result<(), VecslideError>

Available on crate features zip-io and native only.

Packages a source directory into a .vecslide archive (ZIP with renamed extension).

Only available with the native feature (requires std::fs). For in-memory packing (e.g. WASM), use pack_to_writer instead.

Compression strategy:

• manifest.yaml and all .svg files: Deflated
• Audio file: Stored (Opus is already compressed; re-compressing wastes CPU)