test code v1

tests/TC029-flat-plain-medium.md

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+# Test Case: FLAT_PLAIN Medium File
+
+**Test ID**: TC029-flat-plain-medium
+
+## Objective
+
+Verify complete and nonzero-offset reads of a multi-block `FLAT_PLAIN` file.
+
+## Fixture
+
+Use `images/flat.erofs`, `source/flat/medium.dat`, and
+`expected/medium-10-5.bin`. Evidence asserts layout 0 and size 102417.
+
+## Procedure
+
+```sh
+stat -f 'size=%z blocks=%b' "$mnt/medium.dat"
+cmp /tmp/repo22-g1/source/flat/medium.dat "$mnt/medium.dat"
+./read_probe pread "$mnt/medium.dat" 40960 20480 /tmp/tc029-range
+cmp /tmp/repo22-g1/expected/medium-10-5.bin /tmp/tc029-range
+sha256 /tmp/repo22-g1/source/flat/medium.dat "$mnt/medium.dat" \
+  /tmp/repo22-g1/expected/medium-10-5.bin /tmp/tc029-range
+```
+
+Remove the range output, unmount, and detach.
+
+## Expected Results
+
+- Full file and the five-block range match their deterministic sources.
+- No block-boundary truncation or cleanup failure occurs.