Migrate Pathfinding_Austin_Road_Network example off datashader

[brendancol]

Reason or Problem

examples/Pathfinding_Austin_Road_Network.ipynb imports datashader, rasterizes the road network with Canvas().line()/Canvas().points(), and renders with shade/stack/dynspread/set_background.

Proposal

• Remove the datashader imports.
• Replace the Canvas line/point rasterization with arr.xrs.rasterize(...).
• Replace the transfer-function rendering with matplotlib.
• Update any prose that mentions datashader.
• Re-execute the notebook so outputs regenerate cleanly.

Done when

• The notebook runs top to bottom with no datashader import and produces the expected plots.

Part of removing datashader as a dependency; final packaging removal is tracked in #3450.