refactor: colocate cmd tests with their modules

Move tests from the monolithic tests.rs into #[cfg(test)] mod tests
blocks in each command module. Shared test helpers live in
mod.rs::test_helpers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Executed-By: fido

src/command/cmd/core.rs

@@ -275,3 +275,23 @@ pub(super) fn parse_axis(s: &str) -> Result<Axis, String> {
         other => Err(format!("Unknown axis: {other}")),
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn parse_axis_recognizes_all_variants() {
+        assert!(parse_axis("row").is_ok());
+        assert!(parse_axis("column").is_ok());
+        assert!(parse_axis("col").is_ok());
+        assert!(parse_axis("page").is_ok());
+        assert!(parse_axis("none").is_ok());
+        assert!(parse_axis("ROW").is_ok());
+    }
+
+    #[test]
+    fn parse_axis_rejects_unknown() {
+        assert!(parse_axis("diagonal").is_err());
+    }
+}