Both functions previously returned Option despite their invariants
guaranteeing a value: active_view always names an existing view
(maintained by new/switch_view/delete_view), and axis_of only returns
None for categories never registered with the view (a programming error).
Callers no longer need to handle the impossible None case, eliminating
~15 match/if-let Option guards across app.rs, dispatch.rs, grid.rs,
tile_bar.rs, and category_panel.rs.
Also adds Model::evaluate_f64 (returns 0.0 for empty cells) and collapses
the double match-on-axis pattern in tile_bar/category_panel into a single
axis_display(Axis) helper.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
