Folders plugin 6.1100 breaks parent folder view when multibranch project exists — all items invisible in UI

### Jenkins and plugins versions report

Jenkins: 2.555.1
OS: Linux - 6.1.112+
Java: 25 - Eclipse Adoptium (OpenJDK 64-Bit Server VM)

cloudbees-folder:6.1100.ve9eed61d16c4
workflow-multibranch:821.vc3b_4ea_780798
branch-api:2.1280.v0d4e5b_b_460ef
github-branch-source:1967.vdea_d580c1a_b_a_
role-strategy:848.va_a_ea_673cf0b_c

### What Operating System are you using (both controller, and any agents involved in the problem)?

Controller: Linux (GKE COS, amd64), Jenkins 2.555.1 on OpenJDK 25

### Reproduction steps

1. Start with Jenkins 2.555.1 + cloudbees-folder 6.1100
2. Create a Folder (e.g., "MyFolder") containing a regular Pipeline job (e.g., "my-pipeline")
3. Verify "my-pipeline" is visible in the folder UI — it works
4. Create a Multibranch Pipeline inside the same folder (e.g., "my-multibranch") using New Item > Multibranch Pipeline
5. Refresh the folder page — both items are visible initially
6. Restart Jenkins (or wait for any config save that re-serializes the multibranch project)
7. After restart, navigate to the folder — ALL items are invisible in the UI (empty folder view)
8. API still returns both items: /job/MyFolder/api/json?tree=jobs[name] shows both
9. Direct URLs to individual jobs still work
10. Delete the multibranch project — folder immediately shows the remaining items again

Root cause: The multibranch project config.xml serializes with `` inside , , , and elements. After restart, the Folders plugin 6.1100 fails to resolve these relative XML references when rendering the parent folder's AllView, causing all items to become invisible.

Downgrading to cloudbees-folder:6.1079.vc0975c2de294 fixes the issue completely.

### Expected Results

The folder should display all child items (pipelines + multibranch projects) in the UI, same as it did with cloudbees-folder 6.1079.

### Actual Results

The folder shows "This folder is empty" in the UI. The API correctly returns all items. Removing the multibranch project from the folder immediately restores visibility of all other items. The issue persists across restarts and JCasC reloads.

### Anything else?

Workaround: Pin cloudbees-folder to 6.1079.vc0975c2de294 in plugins.txt.

The issue does NOT occur if the multibranch project is created fresh and Jenkins is never restarted. It only occurs after restart when the config.xml is re-read from disk with the `owner reference="../.."` XML back-references.

Upgraded from Jenkins 2.541.2 (folder plugin 6.1079) to 2.555.1 (folder plugin auto-upgraded to 6.1100). All existing folders with multibranch projects broke immediately.

### Are you interested in contributing a fix?

_No response_