Document deferred model producer/publisher architecture

[mark-e-deyoung]

Purpose

Capture the concurred producer/publisher requirement without expanding the current consumer-side MVP.

Decision

• project-produced/fine-tuned models may eventually be published to public, gated/private, OCI-compatible, or provider-native venues;
• publication is a separate producer-side plane from the current selection/acquisition MVP;
• producing projects retain training lineage, project quality acceptance, release intent, and model-specific documentation;
• shared machinery may later provide identity, release gates, venue adapters, credential isolation, idempotent publish/read-back verification, and publication receipts;
• Foundry does not become a universal host by default;
• internally created models may live in the private parts catalog before any publication;
• implementation is deferred until a real model approaches a release decision.

Scope

Documentation only. No publisher implementation, venue selection, credentials, hosting, or external publication.

Tracks #20 and private counterpart SemperSupra/model-artifact-foundry-private#9.

Control correction

An initial documentation write accidentally targeted main; it was immediately reverted and restaged here on a branch/PR. This PR is the intended durable review path.