convert: silent no-op copy when neither --quantize nor --compress specified

[noahgift]

Bug

apr convert model.apr -o output.apr (without --quantize or --compress) silently copies the file with no transformation. The text output even shows Quantization: None (copy) but no error is raised.

Root Cause

In crates/apr-cli/src/commands/convert.rs, lines 63-64 parse both options but there's no validation that at least one is Some:

let quant_type = parse_quantization(quantize)?;    // None
let compress_type = parse_compression(compress)?;  // None
// No check that at least one is Some

The code proceeds to call apr_convert() with ConvertOptions { quantize: None, compress: None }, performing an expensive file copy with no transformation.

Expected Behavior

Should return an error:

Error: At least one of --quantize or --compress must be specified

Fix

Add validation after line 64:

if quant_type.is_none() && compress_type.is_none() {
    return Err(CliError::ValidationFailed(
        "At least one of --quantize or --compress must be specified".into()
    ));
}