[SC-16] Implement distribute_dividends function

[yusuftomilola]

Overview

Implement the dividend distribution system for tokenized assets inside contracts/contrib/.

Location

All work must be inside contracts/contrib/src/dividends.rs.

Acceptance Criteria

• distribute_dividends(env, asset_id, total_amount) — callable by asset tokenizer; iterates all holders; adds (holder_balance / total_supply) * total_amount to each holder's unclaimed_dividends in their OwnershipRecord; emits distribution event
• Panics if revenue_sharing_enabled is false
• get_unclaimed_dividends(env, asset_id, holder) -> i128 — returns holder's unclaimed amount
• Unit test verifying proportional distribution across multiple holders