fix(explore): handle zero-size cursor in binary viewer (#14592)

# Description
Fix cursor panic when handling size zero in binary viewer. Previously,
the cursor would panic
with arithmetic overflow when handling size 0. This PR fixes this by
using `saturating_sub`
to safely handle the edge case of size 0.

Fixes #14589

# User-Facing Changes
- Fixed panic when viewing very small binary inputs in the explore
command (when using `0x[f] | explore`)

# Tests + Formatting
Added tests to verify:
- Cursor handling of size 0
- Safe movement operations with size 0
- Edge case handling with size 1

✅ Verified all checks pass:
- `cargo fmt --all -- --check`
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used`
- `cargo test --package nu-explore --lib cursor`

crates/nu-explore/src/views/cursor/mod.rs

@@ -36,7 +36,7 @@ impl Cursor {
 
     /// The max position the cursor can be at
     pub fn end(&self) -> usize {
-        self.size - 1
+        self.size.saturating_sub(1)
     }
 
     /// Set the position to a specific value within the bounds [0, end]
@@ -121,4 +121,26 @@ mod tests {
         cursor.move_backward(3);
         assert_eq!(cursor.position, 0);
     }
+
+    #[test]
+    fn test_cursor_size_zero_handling() {
+        let cursor = Cursor::new(0);
+        assert_eq!(cursor.end(), 0);
+
+        let mut cursor = Cursor::new(0);
+        cursor.move_forward(1);
+        assert_eq!(cursor.position, 0);
+
+        cursor.move_backward(1);
+        assert_eq!(cursor.position, 0);
+    }
+
+    #[test]
+    fn test_cursor_size_one() {
+        let mut cursor = Cursor::new(1);
+        assert_eq!(cursor.end(), 0);
+
+        cursor.move_forward(1);
+        assert_eq!(cursor.position, 0);
+    }
 }