feat(auction): add Dutch auction mechanism for fractional share sales

[Nanfe01]

Summary

Introduced a Dutch auction system for fractional share sales, enabling time-based price discovery where asset prices decrease linearly until a buyer accepts the current price.

Changes Made

• Added DutchAuction struct containing:

  • token_id
  • shares
  • start_price
  • end_price
  • start_time
  • duration

• Implemented create_dutch_auction(token_id, shares, start_price, end_price, duration) message

• Implemented bid_dutch_auction(auction_id) message allowing buyers to purchase at the current descending price

• Implemented cancel_dutch_auction(auction_id) message (restricted to seller before first bid)

• Added linear price decay formula:

  • current_price = start_price - (elapsed / duration) * (start_price - end_price)

• Auction lifecycle management:

  • Auction completes immediately on first valid bid
  • Remaining shares (if applicable) returned to seller

• Added event emissions:

  • DutchAuctionCreated
  • DutchAuctionBid
  • DutchAuctionCancelled

• Added full unit test coverage for:

  • Auction creation
  • Price decay calculation over time
  • Successful bid execution and auction completion
  • Cancellation before first bid
  • Edge cases across auction states

Auction Behavior

• Price decreases linearly from start_price to end_price over the auction duration
• First successful bid immediately finalizes the auction
• Unsold shares are returned to seller after execution
• Cancellation is only allowed before any bids are placed

Acceptance Criteria

• Add DutchAuction struct
• Add create_dutch_auction(...) message
• Add bid_dutch_auction(...) message
• Implement descending price formula based on elapsed time
• Complete auction on first bid
• Return remaining shares to seller
• Add cancel_dutch_auction(...) message (seller only, pre-bid)
• Emit required events
• Add unit tests for full auction lifecycle

closes #498

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[nanaf6203-bit]

LGTM