update

tests/TC019-48bit-root-nid.md

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+# Test Case: Nonzero 48-bit Root NID
+
+**Test ID**: TC019-48bit-root-nid
+
+## Objective
+
+Verify the `48BIT && rootnid_8b != 0` selector uses the complete 64-bit root
+field and interprets the two-byte union as `blocks_hi`.
+
+## Fixture
+
+Use `images/root8-48bit.erofs`. The structured transformer requires incompat
+bit `0x80`, nonzero image-bounded `rootnid_8b`, `blocks_hi=0`, valid CRC32C,
+and unchanged provider-sized `blocks_lo`. Production fsck.erofs 1.8.6 does not
+recognize this feature; FreeBSD mount/read is mandatory.
+
+## Procedure
+
+Host field check:
+
+```sh
+python3 tests/erofs_fixture.py inspect /work/build/repo22-g1/images/root8-48bit.erofs --path=/root.txt
+od -An -tu2 -j 1038 -N 2 /work/build/repo22-g1/images/root8-48bit.erofs
+od -An -tx4 -j 1104 -N 4 /work/build/repo22-g1/images/root8-48bit.erofs
+od -An -tu8 -j 1136 -N 8 /work/build/repo22-g1/images/root8-48bit.erofs
+```
+
+Mount the same hashed image in FreeBSD, then run:
+
+```sh
+stat -f 'root_ino=%i type=%HT' "$mnt"
+cmp /tmp/repo22-g1/source/compact/root.txt "$mnt/root.txt"
+sha256 /tmp/repo22-g1/source/compact/root.txt "$mnt/root.txt"
+./statfs_probe "$mnt"
+```
+
+## Expected Results
+
+- `feature_incompat & 0x80` is set, `rootnid_8b` is nonzero, and observed root
+  inode equals that complete field.
+- Root content matches the source and statfs uses `blocks_lo` plus zero
+  `blocks_hi`; mount and cleanup succeed.
+- TC150's zero-`rootnid_8b` fallback is a separate selector and is not accepted
+  as evidence for this TC.