# SQ-002: WAL Record Encoding/Decoding

**Status:** `[x] DONE`
**Blocked by:** SQ-001
**Priority:** High

## Description

Implement binary encoding and decoding of individual WAL records, independent of file I/O. Each record is CRC32-protected for corruption detection.

## Files to Create/Modify

- `crates/sq-storage/src/wal/mod.rs` - module declaration
- `crates/sq-storage/src/wal/record.rs` - encode_record, decode_record, CRC validation
- `crates/sq-storage/src/lib.rs` - re-export wal module

## Record Binary Format

```
[crc32: u32]         - CRC32 over everything after this field
[length: u32]        - total byte length of record (excluding crc32 and length)
[offset: u64]        - monotonic offset
[timestamp_ms: u64]  - wall clock millis
[key_len: u32]       - 0 = no key
[key: [u8; key_len]]
[value_len: u32]
[value: [u8; value_len]]
[headers_count: u16]
[for each header:]
  [hdr_key_len: u16]
  [hdr_key: [u8; hdr_key_len]]
  [hdr_val_len: u32]
  [hdr_val: [u8; hdr_val_len]]
```

## Acceptance Criteria

- [ ] `encode_record(&Message) -> Vec<u8>` produces correct binary
- [ ] `decode_record(&[u8]) -> Result<(Message, usize)>` parses correctly (returns bytes consumed)
- [ ] Roundtrip: encode then decode, verify equality
- [ ] Corruption test: flip a byte, decode returns CRC error
- [ ] Edge cases: empty value, empty key, no headers, many headers, max-size value
- [ ] Uses `crc32fast` crate for CRC computation

## Notes

- Use little-endian byte order throughout
- The `length` field allows skipping records without full parsing
- CRC is over the bytes AFTER the CRC field itself