Improve viewer hover highlighting and fix text rendering

[MatthewMckee4]

Summary

• Replace the separate yellow overlay hover highlight with in-place element re-rendering using brighter fill (doubled alpha) and bolder stroke (3px), so the highlight always matches the element's actual shape at every zoom level
• Expand query_point to check a 3×3 neighbourhood of spatial grid cells, eliminating hover detection misses near cell boundaries
• Add edge tolerance (3px / zoom) to Polygon and GdsBox hit tests for more reliable hover near edges
• Fix text and node elements not rendering when they are the only elements in a spatial grid cell — point bboxes produced zero-size cells that were incorrectly culled

Test plan

• Open a GDS file and hover over elements at various zoom levels — highlight should match element shape exactly
• Hover near element edges and cell boundaries — hover should activate reliably
• Open a cell containing only text elements — text should be visible
• Run cargo nextest run -p gdsr-viewer — all 134 tests pass

[codecov]

Codecov Report

❌ Patch coverage is 26.71233% with 107 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/gdsr-viewer/src/drawable.rs	15.62%	81 Missing ⚠️
crates/gdsr-viewer/src/viewport/mod.rs	0.00%	23 Missing ⚠️
crates/gdsr-viewer/src/app.rs	0.00%	3 Missing ⚠️

Files with missing lines	Coverage Δ
crates/gdsr-viewer/src/spatial.rs	99.69% <100.00%> (+0.01%)	⬆️
crates/gdsr-viewer/src/app.rs	0.00% <0.00%> (ø)
crates/gdsr-viewer/src/viewport/mod.rs	48.20% <0.00%> (-2.31%)	⬇️
crates/gdsr-viewer/src/drawable.rs	39.83% <15.62%> (+2.45%)	⬆️

... and 4 files with indirect coverage changes

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