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>
Axis::Unassigned served two purposes that Option already covers:
1. "this category has no assignment yet" → None
2. "this category doesn't exist" → None
By removing the variant and changing axis_of to return Option<Axis>,
callers are forced by the compiler to handle the absent-category case
explicitly (via match or unwrap_or), rather than silently treating it
like a real axis value.
SetAxis { axis: String } also upgraded to SetAxis { axis: Axis }.
Previously, constructing SetAxis with an invalid string (e.g. "diagonal")
would compile and then silently fail at dispatch. Now the type only admits
valid axis values; the dispatch string-parser is gone.
Axis gains #[serde(rename_all = "lowercase")] so existing JSON command
files (smoke.jsonl, etc.) using "row"/"column"/"page" continue to work.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
