feat: NFT marketplace primitive (Seaport-compatible)

[satyakwok]

Deploy a Seaport-fork NFT marketplace on Sentrix mainnet. ERC-721 + ERC-1155 collections list, bid, sell.

Why

NFT marketplace is the third major DeFi primitive after AMM + lending. Without one, NFT projects on Sentrix have to roll their own or send users off-chain. Seaport (OpenSea) is the most battle-tested marketplace contract and is permissively licensed.

Scope

• Fork ProjectOpenSea/seaport and adapt for Sentrix's chainId
• Deploy + verify on mainnet
• Minimal frontend (or document integration so existing tools work)
• Royalty enforcement decision: opt-in / opt-out / mandatory?

Open questions

• Aggregator integration (Blur, Reservoir): worth chasing or wait for organic?
• ERC-721 + ERC-1155 day-1 or 721-only first?

Acceptance

• Verified contracts on mainnet
• One successful sample listing + sale
• Documentation in docs.sentrixchain.com