Add inventories: firm-level inventory buffer with per-sector target ratios

[speedcom]

Summary

• Firm-level inventory buffer with per-sector target ratios (GUS 2024: Mfg 25%, Retail 15%, Agri 30%), spoilage rates, carrying cost, and stress liquidation
• GDP formula gains + ΔInventories (SNA 2008 accounting) — inventory-accelerator amplifies booms, buffers downturns
• 8 env vars (INVENTORY_ENABLED master switch + 7 parameters), 3 output columns (AggInventoryStock, InventoryChange, InventoryToGdp), nCols 175→178
• No new SFC identity — inventory is a real asset (like capitalStock), carrying cost flows through calcPnL

Test plan

• 1075 tests pass (33 new in InventorySpec), 0 failures
• INVENTORY_ENABLED=true FIRM_SIZE_DIST=gus sbt "run 2000 3 inventory_smoke pln" — AggInventoryStock > 0, InventoryToGdp ~0.15, no SFC FAIL
• INVENTORY_ENABLED=false sbt "run 2000 3 baseline pln" — all inventory columns = 0