Prune unused Cartesian axes before rendering

[rosenrot00]

• track Cartesian axes explicitly configured in card YAML or presets
• keep axes referenced by parsed traces, shapes, annotations, grids, or other axes
• prune generated Cartesian axes that are unused before passing the layout to Plotly
• preserve the base xaxis and yaxis

The current defaults materialize up to 30 X-axis and 30 Y-axis layout entries for every card. Plotly processes all of them on every react(), even when a simple chart only uses one X and one Y axis.

Pruning after configuration evaluation is important because entity axis assignments can be dynamic or derived from units. At that point all $fn values and automatic Y-axis assignments are known, so unused defaults can be removed without guessing.

The integration test verifies that a simple parsed chart keeps exactly xaxis and yaxis. Explicit YAML and preset axes, trace references, layout references, and axis-to-axis references remain intact.