web: render highlights in a separate overlay tile layer

[maliberty]

Summary

• Move selection, hover, timing, DRC, route-guide, and flight-line rendering out of base tiles into a dedicated transparent overlay tile layer
• When highlights change, only ~64 overlay tiles are re-requested instead of ~800+ base geometry tiles across all metal layers
• Base tiles stay cached and are only re-rendered on structural changes (visibility toggles, design reload, focus nets)

Changes

Backend:

• generateOverlayTile() renders only overlay shapes on a transparent background
• New "overlay_tile" request type registered in TileHandler
• handleTile() no longer passes highlight data to base tiles
• drawColoredHighlight() draws all colored rects when current_layer is empty (overlay mode)

Frontend:

• createOverlayTileLayer() factory in websocket-tile-layer.js
• Overlay layer created above base layers, below heatmap
• refreshOverlay() / scheduleRefreshOverlay() for highlight-only updates
• Widgets (inspector, timing, DRC, clock-tree, schematic) call refreshOverlay() instead of redrawAllLayers() for highlight changes

[gemini-code-assist]

Code Review

This pull request introduces a lightweight overlay tile layer to render highlight and overlay shapes (such as selection, hover, timing, DRC, and route guides) on a transparent background. This optimization prevents base geometry tiles from being re-rendered when highlights change, significantly improving performance. The review feedback highlights critical issues regarding potential race conditions and memory leaks in the tile layer request handling, a missing default fallback for the refreshOverlay parameter in the inspector panel, and a layer visibility bug where colored highlights are rendered even when their parent layers are hidden.