feat: add Dutch auction mode

[dangelo352]

Fixes #469

Summary

• add Dutch auction mode on unified Auction storage with reserve_price and is_dutch fields
• add start_dutch_auction, get_dutch_price, and buy_dutch with checked linear price decay and reentrancy protection
• settle the first valid Dutch buyer at the current decayed price while respecting payment-token allowlist and marketplace fee split
• document the Dutch auction lifecycle and price formula with a worked example

Tests

• CARGO_TARGET_DIR=/tmp/commitlabs-dutch-auction-target cargo test -p commitment-marketplace dutch -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-dutch-auction-target cargo test -p commitment-marketplace auction -- --nocapture
• git diff --check