[FEATURE] Marketplace catalog and listing foundations

[aernw]

Objective

Prototype the user-facing foundations of a future marketplace or catalog without promising full commerce too early.

Scope

• catalog/listing browsing concepts for templates, presets, or published assets
• listing page structures and trust-building UI states
• clear framing around what is exploratory versus production-ready

Out of Scope

• [Features explicitly not included]
• [Future enhancements to defer]

Acceptance Criteria

• StemHub has a visible path toward catalog-style distribution features
• the product can test marketplace-facing UX without overcommitting on full commerce

Dependencies

Blocked by: (none)

Blocks: (none)

Success Metrics

• [How to measure success - optional]

Roadmap Context

Milestone: M7 - Launch Readiness & Cross-DAW Expansion
Parent workstream(s): #163, #214