feat: implement NFT minting factory with royalty support

[internet-dot]

Summary

Closes #21

Adds contracts/nft_factory/ — a Soroban smart contract for minting NFTs with configurable royalty support.

Features

• Metadata URI support — each token stores an off-chain URI (e.g. IPFS)
• Configurable royalties — per-token or admin-set default (basis points, 0–10000)
• Batch minting — batch_mint creates multiple tokens in a single call
• Admin controls — set default royalty recipient and percentage

Files

• contracts/nft_factory/Cargo.toml
• contracts/nft_factory/src/lib.rs
• contracts/nft_factory/src/test.rs (4 tests)

[internet-dot]

Closing — this contribution was made in error.