feat: Implement Dutch auction mechanism for fractional share sales

[nanaf6203-bit]

Description

Currently fractional shares can only be sold at fixed prices via listings. Add a Dutch auction mechanism where the price decreases over time until a buyer accepts.

Acceptance Criteria

• Add DutchAuction struct: token_id, shares, start_price, end_price, start_time, duration
• Add create_dutch_auction(token_id, shares, start_price, end_price, duration) message
• Add bid_dutch_auction(auction_id) message — buyer pays current descending price
• Current price = start_price - (elapsed/duration) * (start_price - end_price)
• Auction completes on first bid; remaining unsold shares returned to seller
• Add cancel_dutch_auction(auction_id) message (seller only, before any bid)
• Emit DutchAuctionCreated, DutchAuctionBid, DutchAuctionCancelled events
• Add unit tests for all auction lifecycle states

[Nanfe01]

@Nanfe01 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

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Congratulations, @Nanfe01! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

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