feat: add Dutch descending-price auction mode

[olaleyeolajide81-sketch]

Summary

Add Dutch auction mode for faster default liquidation recovery with overflow-safe monotone decreasing price computation.

Changes Made

• Added AuctionMode enum (English/Dutch) to types.rs
• Extended AuctionConfig with Dutch auction parameters (dutch_start_price, dutch_floor_price)
• Implemented compute_dutch_price function for overflow-safe monotone decreasing price calculation
• Updated init_auction to accept mode and Dutch parameters with validation
• Updated place_bid to support Dutch auction first-bid settlement
• Added comprehensive Dutch auction tests covering price at start, mid, and floor
• Updated existing tests to use new init_auction signature
• Fixed duplicate members in Cargo.toml

Test plan

• Price at start: Dutch auction accepts bid at dutch_start_price and closes immediately
• Price at mid: Dutch auction price decays linearly, accepts bid at calculated midpoint price
• Price at floor: Dutch auction clamps at dutch_floor_price, accepts bid at floor price
• First bid settlement: Dutch auction closes on first qualifying bid, rejects subsequent bids
• Price validation: Bids below current Dutch price are rejected
• Parameter validation: Dutch auctions require start_price and floor_price with start_price >= floor_price
• English mode unchanged: English auctions maintain existing behavior with new signature
• Min bid enforcement: Dutch auctions respect min_bid requirement in addition to current price

Generated with Devin https://cli.devin.ai/docs

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#351	Add Dutch-auction descending-price mode to auction_contract for faster default recovery	Link to this issue

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