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/view/axis.rs

@@ -1,21 +1,19 @@
 use serde::{Deserialize, Serialize};
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "lowercase")]
 pub enum Axis {
     Row,
     Column,
     Page,
-    /// Not yet assigned
-    Unassigned,
 }
 
 impl std::fmt::Display for Axis {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            Axis::Row => write!(f, "Row ↕"),
+            Axis::Row    => write!(f, "Row ↕"),
             Axis::Column => write!(f, "Col ↔"),
-            Axis::Page => write!(f, "Page ☰"),
-            Axis::Unassigned => write!(f, "─"),
+            Axis::Page   => write!(f, "Page ☰"),
         }
     }
 }