Quant: Universe + core types

Define the asset universe and core types for the quant system.

Files: Omni/Fund/Quant/Universe.hs

Deliverables:

• Asset type with ticker, name, sector, asset type, tradeable flag
• Sector enum (11 GICS sectors + CrossAsset)
• AssetType enum (ETF, Stock, Commodity, Index)
• sectorETFs list (XLK, XLV, XLF, XLE, XLI, XLY, XLP, XLC, XLRE, XLB, XLU)
• crossAssets list (SPY, TLT, GLD, BTC-USD, VIX)
• defaultUniverse combining both
• Aeson instances for all types
• Basic tests

This is the foundation — every other module imports Universe.