Routing V2: incremental grid router in a web worker

[milanofthe]

Replaces the edge router with an incremental engine that runs in a web worker. Port handles and the 10px grid are unchanged.

• Orthogonal A* on the grid: node interiors impassable, margins and port stubs passable at extra cost
• Negotiated congestion keeps different nets off shared grid lines; fan-out shares a trunk
• Only affected connections are rerouted after changes; routes are published per connection
• Waypoints are routed optimally as a whole, no loops around waypoints
• Canvas nodes and edges use raw state, edge highlights are computed centrally
• Zoom-out limit lifted, assembly animation capped at 2 s
• Old router and unused pathfinding dependency removed
• Vitest added with routing invariants, scenario generator and benchmark (npm test, npm run bench:routing)

Measurements with 1000 connections:

• Engine, full routing: about 0.2 s, no overlap between nets (old router: 26 s)
• Production build, headless Chrome: load 3.8 s, block drag median 16.7 ms per frame, hover 16.7 ms (before: 50 s load and 4 s per frame while dragging, dev build)