# Test Case: 48-bit Block Count Parsing

**Test ID**: TC017-48bit-blocks-parse
**Category**: Filesystem Metadata
**Priority**: Medium

## Objective

Verify the Linux EROFS union rule dynamically: with incompat bit `0x80` and a
nonzero `rootnid_8b`, combine `blocks_lo` with `rb.blocks_hi << 32` and preserve
the complete count through mount.

## Fixture Qualification

```sh
work=$(mktemp -d /tmp/repo22-tc017.XXXXXX)
tests/prepare_error_fixtures.sh "$work/fixtures"
python3 tests/erofs_fixture.py inspect \
  "$work/fixtures/48bit-statfs-prefix.erofs"
od -An -tx2 -j 1038 -N 2 "$work/fixtures/48bit-statfs-prefix.erofs"
od -An -tx4 -j 1060 -N 4 "$work/fixtures/48bit-statfs-prefix.erofs"
od -An -tx8 -j 1136 -N 8 "$work/fixtures/48bit-statfs-prefix.erofs"
grep '^48bit-statfs-prefix ' "$work/fixtures/fixture-evidence.txt"
```

Require `blocks_hi=1`, `rootnid_8b != 0`, a valid recomputed CRC, and a recorded
prefix SHA256. The expected count is `blocks_lo | (1 << 32)`.

## FreeBSD Procedure

Create and attach the sparse provider exactly as in TC010. Record `diskinfo`,
mount it read-only, compare `df -k` total with `total_blocks * 4`, read
`control.txt`, then unmount and detach the dynamic md unit.

Also attach the unextended small prefix once and require mount failure with
`ENXIO`; record this only as the negative media-size guard, not the positive
parse result.

## Expected Results

The sparse provider mount succeeds and reports the full nonzero-high-word count.
The small prefix fails `ENXIO`. If no qualifying provider can be attached,
record the metadata parsing evidence separately and mark the dynamic portion
SHELVED, never PASS.
