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tests/TC135-prefix-metabox-and-primary-fallback.md

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+# Test Case: Long-prefix backing selection
+
+**Test ID**: TC135-prefix-metabox-and-primary-fallback
+**Fixtures**: `metabox-plain.erofs`, `basic.erofs`, `prefix-primary.erofs`
+
+## Objective
+
+Verify prefix table selection in order: metabox, packed inode, then primary
+metadata. The manifest must prove each backing and record all changed super
+fields and image hashes.
+
+## Manual steps
+
+Use a separate attach/mount/cleanup cycle for each image:
+
+```sh
+# metabox-plain.erofs
+lsextattr user /mnt/repo22-g4/metabox-a
+getextattr -qq -x user repo22.application.component.setting \
+  /mnt/repo22-g4/metabox-a
+
+# basic.erofs (packed backing)
+lsextattr user /mnt/repo22-g4/prefix-user-0
+getextattr -qq -x user repo22.application.component.setting \
+  /mnt/repo22-g4/prefix-user-0
+
+# prefix-primary.erofs (PLAIN_XATTR_PFX)
+lsextattr user /mnt/repo22-g4/prefix-user-0
+getextattr -qq -x user repo22.application.component.setting \
+  /mnt/repo22-g4/prefix-user-0
+```
+
+## Expected results
+
+Values are `long-prefix-value`, `prefix-value-0`, and `prefix-value-0`.
+Every cycle cleans its mount and md provider.