Add Variance Charts and IBCS Theme

Added

• IBCS Integrated Variance Column Chart - Two-layer chart with embedded absolute variance
• IBCS Integrated Variance Column Chart - DL Compatible - DL-optimized variant
• Variance Line Chart - Analytical line chart with reference baseline and variance indicators
• Variance Line Chart - DL Compatible - DL optimized variant
• Enhanced template documentation and setup guides
• IBCS theme file (Theme_IBCS.json) for consistent visual styling

Changed

• Improved parameter system with DAX DATATABLE approach
• Refined Direct Lake compatibility approach