feat(core): Add AWT-based font measurement for accurate autofit #156

Summary

Add pixel-accurate column width calculation using Java AWT font metrics, similar to Apache POI's autoSizeColumn implementation.

Background

PR #155 added basic autofit using charCount * 0.85 + 1.5 heuristic. This works but produces widths ~5-10% wider than Excel's actual autofit.

Proposed Solution

• Create FontMetrics utility using AWT TextLayout + FontRenderContext
• Graceful fallback to heuristic in headless environments
• Style-aware: respects font family, size, bold/italic from cell styles

Technical Approach (POI-style)

• Excel uses "character units" based on max digit width of default font
• Calibri 11pt at 96 DPI: max digit width ≈ 7 pixels
• Formula: width = (pixels + 5) / maxDigitWidth
• +5 padding: 2px left margin + 2px right margin + 1px gridline

Implementation

1. New file: xl-core/src/com/tjclp/xl/util/FontMetrics.scala
2. Integrate into WriteCommands.calculateAutoFitWidth()
3. Add tests for AWT and fallback modes

Acceptance Criteria

• AWT measurement within 5% of Excel's autofit
• Fallback works in headless/CI environments
• All existing autofit tests pass
• Performance: <100ms for 1000 cells