[ADD] pos_lot_selection_stock_aware: Stock-aware lot selection in POS

[nkirui]

This module extends pos_lot_selection to provide stock-aware lot selection in Point of Sale:

• Display available quantity beside lot numbers (e.g., LOT001 (Qty: 15))
• Auto-filter lots with zero quantity at POS location
• Location-aware filtering using POS picking type source location
• Prevent typing invalid lot numbers (selection-only interface)
• Real-time quantity updates via RPC calls