Add support for additional draft checkpoint

[bitgamma]

• Added support for draft checkpoint in llamacpp recipe. This allows downloading a drafter model (MTP, EAGLE3, etc) alongside the main model. Works well with the existing mtp label and with manually specifying MTP/drafting options
• Added Gemma4 MTP models that use this mechanism.

[fl0rianr]

Nice addition, great! One concern: --model-draft is reserved unconditionally, even when draft_path is empty. Since validate_custom_args() rejects reserved flags, this prevents users from manually passing --model-draft / -md / --spec-draft-model through llamacpp_args for models that do not have a registered draft checkpoint. Could we move the reservation into the if (!draft_path.empty()) block, e.g. pass the aliases directly to push_arg() when Lemonade actually adds the managed --model-draft argument?

[bitgamma]

@fl0rianr, this is intentional and the same is done with mmproj file and all other reserved argumens really. Allowing specifying arbitrary paths is a potential security risk

[fl0rianr]

Your right - I remember that topic brought up by Mario. In that case you have my approval.

[fl0rianr]

not possible to merge until the whisper issue is resolved. https://discord.com/channels/1392562559122407535/1394414791593820252/1517829183206854736