feat: multi-recipient royalty splits for collaborative beats

[brightpixel-dev]

Difficulty: Hard · Priority: Critical

Overview

Support multiple collaborators (co-producer, vocalist, etc.) splitting earnings on one sample automatically on-chain. Today SampleData has a single uploader and earnings are paid to one address only.

Acceptance Criteria

• Upload form supports adding N collaborator addresses + percentage shares (must sum to 100%)
• Contract stores splits alongside the sample and pays each collaborator's share on purchase
• Profile page shows "your split" per sample instead of assuming full ownership
• Withdraw flow works per-collaborator

Technical Approach

Requires a new/updated contract entry point (crate-contracts) accepting Vec<(Address, u32 bps)>, crediting each recipient's earnings balance pro-rata on purchase_license. Frontend: dynamic collaborator rows in Upload with live remaining-% validation.

Spans two repos (contract + frontend) and changes the earnings model everywhere it's read (Profile, withdraw flow) — not a self-contained UI addition.