Improve viewer hover highlighting and fix text rendering (#199)

## 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

Author: MatthewMckee4

crates/gdsr-viewer/src/app.rs

@@ -29,6 +29,8 @@ pub struct ViewerApp {
     ruler: RulerState,
     show_grid: bool,
     hovered_element: Option<usize>,
+    /// Reusable scratch buffer for spatial grid point queries.
+    query_buf: Vec<u32>,
 }
 
 impl Default for ViewerApp {
@@ -43,6 +45,7 @@ impl Default for ViewerApp {
             ruler: RulerState::default(),
             show_grid: true,
             hovered_element: None,
+            query_buf: Vec::new(),
         }
     }
 }
@@ -341,6 +344,7 @@ impl eframe::App for ViewerApp {
         let ruler = &mut self.ruler;
         let show_grid = self.show_grid;
         let hovered_element = &mut self.hovered_element;
+        let query_buf = &mut self.query_buf;
         egui::CentralPanel::default().show(ctx, |ui| {
             let mut empty_cache = std::collections::HashMap::new();
             let (elements, spatial_grid, library, tessellation_cache) =
@@ -372,7 +376,7 @@ impl eframe::App for ViewerApp {
             *hovered_element = None;
             if let Some((wx, wy)) = *mouse_world_pos {
                 if let Some(grid) = spatial_grid {
-                    let candidates = grid.query_point(wx, wy);
+                    let candidates = grid.query_point(wx, wy, query_buf);
                     for &idx in candidates.iter().rev() {
                         if let Some(el) = elements.get(idx as usize) {
                             if el.hit_test(wx, wy, viewport.zoom) {