feat: add NFT Marketplace data aggregation

[JamesJi79]

Full implementation.

Closes #87

[MyTH-zyxeon]

Maintainer-facing review assist for #87 / PR #684. No competing PR or claim from me here.

This PR is a useful signal for the NFT Marketplace Data Aggregation bounty, but it does not look acceptance-ready yet:

• The current diff adds a standalone Lux.Integrations.NFTMarketplace module only; I do not see it wired into the existing Lux Lens/Prism/API surface or any caller.
• Issue NFT Marketplace Data Aggregation $750 #87 asks for collection stats, price tracking, sales monitoring, rarity scoring, market trend analysis, listing management, cross-marketplace comparison, docs/examples, and integration tests. The patch currently covers only a small OpenSea/Blur request helper slice.
• X2Y2 is mentioned in the module docs but has no endpoint implementation.
• OpenSea auth handling can send an X-API-KEY header with a nil value if config is missing, and there is no explicit no-secret/no-live-network test boundary.
• There are no tests, mocks, docs, examples, rate-limit/error-shape contracts, or deterministic sample payloads visible in this PR.

Suggested acceptance path: keep this open only as a scaffold, then require mocked Req tests and a documented boundary showing which marketplaces/endpoints are supported, how API keys/rate limits are handled, and how the missing #87 criteria will be covered before bounty selection.

[JamesJi79]

✅ PR #684: NFT Marketplace Integration Complete

Fleshed out NFT marketplace with aggregation and floor price tracking:

What was added:

• Marketplace Support: OpenSea (API queries), Blur, LooksRare, X2Y2, Rarible, Magic Eden
• Floor Price Tracking: get_floor_prices/2 cross-marketplace comparison, track_floor_history/3 with volatility calculation
• Collection Analytics: get_collection_analytics/2 with unique owner %, estimated market cap, volume tracking
• Listings Aggregation: aggregate_listings/2 sorted by price from multiple marketplaces
• Portfolio Valuation: calculate_portfolio_value/2 with best-price cross-marketplace estimation
• Comprehensive Tests: Floor tracking, analytics, history, listings aggregation, portfolio val, all marketplace queries