feat: add NFT Marketplace contract with auctions, offers, and royalty enforcement

[uadamu00005-netizen]

Closes #62

Summary

Implements a comprehensive NFT marketplace module enabling P2P NFT trading, English and Dutch auctions, royalty enforcement, and collection management with integrated payment settlement.

Changes

New Contract (contracts/nft-marketplace/)

• Fixed-price listings with configurable currency support and payment escrow
• English auctions with bid increments, auto-extension on late bids, and hammer-down settlement
• Dutch auctions with linear time-based price decay to a configurable floor
• Royalty enforcement at transaction level with per-creator split configuration
• Offer-based trading with time-limited make/accept/cancel flows
• Collection management with IPFS metadata pinning and on-chain verification
• Bulk operations for atomic batch listing and auction creation (up to 50 items)
• Marketplace fees with governance-controlled platform/creator/collection splits

Shared Crate Updates

• Added 9 NFT-specific error codes (range 2000–2008)
• Added 8 marketplace event emitters (nft_listed, nft_sold, nft_bid, nft_auction, nft_settle, nft_offer, royalty_paid, collection_registered)

Key Design Decisions

• Separate MarketError enum — Soroban's #[contracterror] macro limits enums to 50 variants; marketplace errors live in their own enum to avoid exceeding the shared crate's limit
• Sentinel address pattern — Auction current_bidder uses the contract's own address as a "no bids yet" placeholder, checked via current_bid > 0
• Checks-effects-interactions — All state mutations precede token transfers for atomic safety
• Lazy royalty evaluation — Royalties computed and distributed only at sale time, not at listing time

Test Coverage

31 unit tests covering:

• Initialization, config, and currency whitelist management
• Collection registration with royalty splits
• Full listing lifecycle (list → buy → verify balances and fee distribution)
• English auction with multiple bidders, bid validation, and auto-extension
• Dutch auction price decay, buy, and cancel
• Offer lifecycle (make → accept, make → cancel, expiry handling)
• Royalty distribution to multiple creators
• Bulk listing and auction creation
• Edge cases (zero price, bad metadata hash, non-whitelisted currency, floor ≥ start)

Acceptance Criteria

• Fixed-price listings allow immediate purchase at specified price
• English auctions properly track bids with current highest bidder
• Auction auto-extends if bid received in final minutes
• Dutch auctions show accurate current price based on time decay
• Royalties deducted and distributed to creators per royalty split
• NFT transferred to buyer only after payment confirmed
• Payment escrow holds funds until transfer completion
• Collection metadata stored and retrievable with verified IPFS hashes
• Offers can be created, accepted, or cancelled within time windows
• Bulk operations execute atomically for all items
• Marketplace fees deducted accurately and distributed per splits
• 100% unit test coverage including auction edge cases (multiple bidders, extensions)