turbo-persistence: add CRC32 block checksums (#90754)
### What?
Add a 4-byte CRC32 checksum to every block in turbo-persistence SST
files.
### Why?
Detect on-disk cache corruption early with a clear error message, rather
than silently returning wrong data or hitting confusing LZ4
decompression failures.
### How?
**New on-disk block format:**
```
[4B uncompressed_length] [4B CRC32 checksum] [block data]
```
(Previously the checksum field did not exist — header was just the 4B
uncompressed_length.)
- The checksum is computed on the **uncompressed** block data using
`crc32fast` (hardware-accelerated CRC32C on modern CPUs).
- On write, the checksum is computed before compression and stored in
the block header.
- On read, the checksum is verified after decompression (or directly for
uncompressed blocks). A mismatch returns an `anyhow` error with a
message like: `Cache corruption detected: checksum mismatch in block N
of SST file seq:M`.
- During compaction, medium values are copied as raw compressed blocks
without decompression — the checksum is carried through the
`MediumRaw`/`LazyLookupValue::Medium` path unchanged.
- A `BLOCK_HEADER_SIZE` constant (= 8) replaces magic numbers across the
write, read, and inspect code.
**Files changed:**
- `compression.rs` — `checksum_block()` helper wrapping
`crc32fast::hash()`
- `static_sorted_file_builder.rs` — write path
(`write_raw_block_to_file`, `write_block_to_file`, `close()` index
block), `EntryValue::MediumRaw` carries checksum
- `static_sorted_file.rs` — read path (`get_raw_block_slice`,
`read_block` with `verify_checksum()` helper)
- `lookup_entry.rs` — `LazyLookupValue::Medium` carries checksum for
compaction
- `bin/sst_inspect.rs` — updated block parsing offsets
- `README.md` — documented new format
**Tests:**
- All 48 existing tests pass (checksums are transparent to correct data)
- 2 new corruption tests: one corrupts a compressed block's checksum,
the other corrupts an uncompressed block's data — both verify the error
is returned
