test code v1

tests/TC021-compact-inode-nlink1.md

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+# Test Case: Compact Inode I_NLINK_1
+
+**Test ID**: TC021-compact-inode-nlink1
+
+## Objective
+
+Verify bit 4 on a non-directory compact inode forces `st_nlink=1` while the
+`i_nb` union remains available for address high bits.
+
+## Fixture
+
+Use `images/compact-nlink1.erofs` and `source/compact/single.txt`.
+`fixture-evidence.txt` asserts a 32-byte regular inode, bit 4 set,
+`i_nb=0x1234`, and valid recomputed CRC32C.
+
+## Procedure
+
+```sh
+stat -f 'ino=%i nlink=%l size=%z mode=%p' "$mnt/single.txt"
+cmp /tmp/repo22-g1/source/compact/single.txt "$mnt/single.txt"
+sha256 /tmp/repo22-g1/source/compact/single.txt "$mnt/single.txt"
+```
+
+## Expected Results
+
+- `st_nlink=1` despite on-disk `i_nb=0x1234`.
+- Inline file data remains source-exact; mount and cleanup succeed.
+- An `i_nb` value of zero is not used as a substitute for the bit-4 encoding.