Clarify semantic contracts and publish the Contact pack

[callumalpass]

Summary:

• explain local types, field mappings, contract settings, multi-contract types, and multi-application consumption
• publish the semantic mdbase.contact catalog entry and friendly Contact type pack while removing JSContact from default discovery
• publish the shared-type/multiple-contracts testbed scenario and pin all source repositories to merged commits

Validation:

• pnpm sync:sources against the pinned local source revisions
• pnpm test (Astro check, production build, specification import, local link check)
• live visual review of the contracts catalog at desktop width