fix: use CRC-32C (Castagnoli) instead of IEEE for WAL (C1)

Per design §3.2 line 359, all WAL CRC computations must use crc32c
(Castagnoli polynomial 0x82F63B78). The previous code used crc32.IEEE
(Ethernet/PNG polynomial 0xEDB88320) in 5 places. While the system was
self-consistent (encode + decode both used IEEE), it diverged from the
design spec and lost SSE4.2 hardware acceleration (the native CRC32
instruction only supports Castagnoli).

Changes:
- wal/crc32c.go (new): package-level crc32cTable = crc32.MakeTable(
  crc32.Castagnoli). Central definition prevents future drift.
- wal/header.go: 2 ChecksumIEEE calls replaced with crc32.Checksum(
  data, crc32cTable). Comment updated to reference design §3.2 line 341, 359.
- wal/record.go: 2 ChecksumIEEE calls replaced.
- wal/block_writer_test.go: 1 ChecksumIEEE call in test helper replaced.
- wal/crc32c_test.go (new): 4 regression guards:
  - TestCRC32CStandardVector: RFC 3720 fixed vector (crc32c("123456789")
    = 0xE3069283).
  - TestCRC32CEdistinctFromIEEE: confirms IEEE produces different value.
  - TestHeaderCRCUsesCastagnoli: direct assertion on stored header CRC
    (catches paired encode/decode reversion that round-trip tests miss).
  - TestPhysicalRecordCRCUsesCastagnoli: same for physical record CRC.

BREAKING CHANGE: WAL files written before this fix (with IEEE CRC)
cannot be read after this fix (expects crc32c). Phase 1 has not been
released, so no real data migration is needed. Production users
post-release would need to drain + re-create the database.

Developers pulling this change should delete any local Phase-1 WAL
directories (`rm -rf <db-dir>/segment-*.wal`) before running the code;
old IEEE-encoded WALs will fail recovery on local dev machines.

Verified: all existing round-trip tests pass (encode + decode both use
crc32c, still self-consistent). Full suite green including
go test -race ./... .

Audit context: docs/audit-3.2.md C1.

wal/record.go

@@ -26,8 +26,9 @@ func EncodePhysicalRecord(recType uint8, payload []byte) []byte {
 	buf[6] = recType
 	copy(buf[7:], payload)
 
-	// CRC covers bytes [4:] = length + type + payload.
-	crc := crc32.ChecksumIEEE(buf[4:])
+	// CRC covers bytes [4:] = length + type + payload. Castagnoli polynomial
+	// per design §3.2 line 359.
+	crc := crc32.Checksum(buf[4:], crc32cTable)
 	binary.LittleEndian.PutUint32(buf[0:4], crc)
 
 	return buf
@@ -52,7 +53,7 @@ func DecodePhysicalRecord(data []byte) (rec *PhysicalRecord, consumed int, err e
 	copy(payload, data[7:7+length])
 
 	// Verify CRC: covers length + type + payload.
-	expectedCRC := crc32.ChecksumIEEE(data[4 : 7+length])
+	expectedCRC := crc32.Checksum(data[4 : 7+length], crc32cTable)
 	if crc != expectedCRC {
 		return nil, 0, errors.New("record: CRC mismatch")
 	}