[FEATURE] Fork and remix UX for public projects

[aernw]

Objective

Turn public projects into remixable starting points in the web product.

Scope

• fork/remix entry points on public projects
• user-facing lineage and source attribution surfaces
• clear states around what happens when a user forks a public project

Out of Scope

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

Acceptance Criteria

• public remixing is understandable and attractive from the UI
• users can see where a remix came from and where it went

Dependencies

Blocked by: (none)

Blocks: (none)

Success Metrics

• [How to measure success - optional]

Roadmap Context

Milestone: M6 - Productization & Coverage Expansion
Parent workstream(s): #163, #211