Add SVG export for cells and libraries

[MatthewMckee4]

Summary

• Add cell_to_svg and library_to_svg public functions to the gdsr crate
• Render polygons, paths, boxes, text, and nodes as SVG elements with layer-based coloring (same 16-color palette as viewer)
• Flatten cell references recursively so nested hierarchies render correctly
• Auto-size viewport to bounding box with 5% margin
• Flip Y axis (GDS Y-up → SVG Y-down) via SVG transform group
• Escape text content for XML safety

Closes #134

Test plan

• 11 new snapshot tests covering each element type, reference flattening, multi-layer colors, XML escaping, empty cells, and Y-flip transform
• Full test suite passes (691 tests)
• All pre-commit checks pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
crates/gdsr/src/utils/svg.rs	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[codspeed-hq]

Merging this PR will degrade performance by 23.08%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

❌ 1 regressed benchmark
✅ 3 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	write_medium	784.8 µs	1,020.3 µs	-23.08%

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_{Comparing feat/svg-export (e093c0f) with main (327f160)}