This change improves superblock error handling and integrity checking:

- Fail fast on IO errors during superblock read instead of treating them as
  fresh/unformatted disks
- Add footer superblock validation for HDD devices to detect corruption
- Check header and footer write errors independently to prevent silent failures
- Add comprehensive unit tests covering all error scenarios

Changes:
- read_first_block(): Now throws std::system_error on IO errors instead of
  returning garbage data
- write_super_block(): Separately validates header and footer writes with
  independent error checking
- sanity_check(): New method that validates footer consistency on HDD devices
  by comparing header and footer superblocks using full memcmp
- Added 6 unit tests covering IO errors, corruption detection, and footer
  validation scenarios

Use release assert instead of exceptions for superblock IO errors

Per review feedback, replace exception throws with HS_REL_ASSERT for all
superblock IO errors to ensure immediate crash on failure:

- read_first_block(): Use HS_REL_ASSERT instead of throwing std::system_error
- sanity_check(): Use HS_REL_ASSERT for header/footer read errors and mismatch
- Update tests from ASSERT_THROW to ASSERT_DEATH to verify crash behavior

On first boot (format_devices path), all devices have invalid headers on
disk when PhysicalDev is constructed — before write_super_block is called.
The original code blindly memcmp'd header vs footer on garbage data,
causing a crash on HDD devices where mirror_super_block=1.

In load_devices, a device with an invalid header goes to pdevs_to_format
and then format_single_device. At that point m_first_blk_hdr (and thus
m_pdev_info.system_uuid) is already populated from other valid devices in
the cluster. So if the footer on disk is valid and its system_uuid matches,
it means the footer survived but the header was corrupted — not a fresh
device. If the footer uuid does not match (or footer is invalid), it is
leftover/garbage data and safe to treat as first boot.

New logic:
- Header valid: compare header and footer as before
- Header invalid, footer valid + uuid matches cluster: header corruption, assert
- Otherwise: first boot / leftover data, skip validation