composefs/status: derive spec from staged deployment when present

The ostree backend derives `host.spec.image` from the staged deployment
first, falling back to booted. This ensures the spec reflects the
desired state after a `bootc switch`.

But the composefs backend on the other hand was only deriving it from
the booted deployment, which meant the spec never reflected a staged
switch.

Fix the composefs logic to match the ostree one.

Heavily AI-guided.

Signed-off-by: Jonathan Lebon <jonathan@jlebon.com>

Author: jlebon

crates/lib/src/bootc_composefs/status.rs

@@ -890,7 +890,6 @@ async fn composefs_deployment_status_from(
         };
 
         if verity_digest == booted_composefs_digest.as_ref() {
-            host.spec.image = boot_entry.image.as_ref().map(|x| x.image.clone());
             host.status.booted = Some(boot_entry);
             continue;
         }
@@ -908,6 +907,15 @@ async fn composefs_deployment_status_from(
         extra_deployment_boot_entries.push(boot_entry);
     }
 
+    // see similar logic in the ostree-version of this
+    host.spec.image = host
+        .status
+        .staged
+        .as_ref()
+        .or(host.status.booted.as_ref())
+        .and_then(|entry| entry.image.as_ref())
+        .map(|img| img.image.clone());
+
     // Shouldn't really happen, but for sanity nonetheless
     let Some(boot_type) = boot_type else {
         anyhow::bail!("Could not determine boot type");