blockdev: Add Device::root_disk() to find the whole-disk ancestor

Add a root_disk() method that walks the parent chain via lsblk
--inverse to find the root (whole disk) device, then refreshes it
to populate its actual children (partitions).

This is needed by callers that start from a partition device (e.g.
from list_dev_by_dir on /sysroot) and need to find sibling
partitions such as the ESP.

Assisted-by: Claude Code (Opus 4)
Signed-off-by: ckyrouac <ckyrouac@redhat.com>

Author: ckyrouac

crates/blockdev/src/blockdev.rs

@@ -184,6 +184,37 @@ impl Device {
         }
     }
 
+    /// Walk the parent chain to find the root (whole disk) device.
+    ///
+    /// Returns the root device with its children (partitions) populated.
+    /// If this device is already a root device, returns a clone of `self`.
+    /// Fails if the device has multiple parents at any level.
+    pub fn root_disk(&self) -> Result<Device> {
+        let Some(parents) = self.list_parents()? else {
+            // Already a root device; re-query to ensure children are populated
+            return list_dev(Utf8Path::new(&self.path()));
+        };
+        let mut current = parents;
+        loop {
+            anyhow::ensure!(
+                current.len() == 1,
+                "Device {} has multiple parents; cannot determine root disk",
+                self.path()
+            );
+            let mut parent = current.into_iter().next().unwrap();
+            match parent.children.take() {
+                Some(grandparents) if !grandparents.is_empty() => {
+                    current = grandparents;
+                }
+                _ => {
+                    // Found the root; re-query to populate its actual children
+                    parent.refresh()?;
+                    return Ok(parent);
+                }
+            }
+        }
+    }
+
     /// Return this device's children, querying lsblk if not already populated.
     ///
     /// Devices obtained from a parent (inverse) chain lack children;