refactor: remove Axis::Unassigned; axis_of returns Option<Axis>

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>

src/command/types.rs

@@ -1,4 +1,5 @@
 use serde::{Deserialize, Serialize};
+use crate::view::Axis;
 
 /// All commands that can mutate a Model.
 ///
@@ -44,8 +45,7 @@ pub enum Command {
     SwitchView { name: String },
 
     /// Set the axis of a category in the active view.
-    /// axis: "row" | "column" | "page"
-    SetAxis { category: String, axis: String },
+    SetAxis { category: String, axis: Axis },
 
     /// Set the page-axis selection for a category.
     SetPageSelection { category: String, item: String },