test code v1

tests/TC106-zstd-level15.md

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+# Test Case: zstd Compression Level 15
+
+**Test ID**: TC106-zstd-level15  
+
+**G5 fixture contract**: Use [G5-MANUAL-SETUP.md](G5-MANUAL-SETUP.md).
+Its generated paths, source comparisons, structured corruption offsets, and
+cleanup rules supersede placeholder examples in this file.
+**Category**: Compression  
+**Priority**: High  
+**Regression**: None
+
+## Objective
+Verify zstd compressed data path with level 15 (high) compression.
+
+## Preconditions
+- EROFS image created with zstd level 15: `mkfs.erofs -zzstd,level=15 test.img src/`
+- Large test files for compression testing
+- FreeBSD 13.0+ with libzstd
+
+## Test Steps
+1. Create image with zstd level 15 compression
+2. Attach and mount: `unit=$(mdconfig -a -t vnode -f test.img); mount -t erofs -o ro /dev/${unit} /mnt`
+3. Read all test files
+4. Verify content integrity
+5. Test partial reads
+
+## Expected Results
+- All files decompress correctly
+- Better compression than level 1
+- Decompression still fast
+- No memory issues
+
+## Verification Method
+- All checksums match
+- Compression ratio > zstd level 1
+- Decompression works for all files
+- Random access validated
+
+## Cleanup
+```sh
+umount /mnt
+mdconfig -d -u "${unit}"
+```
+
+## Notes
+Tests feature 59: zstd compressed data path with high compression.