feat: initialize project structure and error types

- go.mod with github.com/dailz/go-kv, Go 1.26.3, testify
- config/config.go with WalConfig, Validate() with checked arithmetic
- errors.go with sentinel errors (ErrCommitUnknown, ErrWriteStopped, etc.)
- wal/constants.go with all WAL format constants and enums
- wal/header.go with WAL File Header encode/decode (CRC32 IEEE)
- wal/record.go with Physical Record codec, block boundary, SplitIntoRecords
- wal/entry.go with WAL Entry codec (varint keys/values, OpType, ValueKind)
- wal/sequence.go with SequenceManager (atomic, CAS, overflow-safe)
- manifest/manifest.go with MANIFEST stub (Load/Save atomic)
- manifest/current.go with CURRENT file (WriteCurrent/ReadCurrent)
- Comprehensive tests for all modules
- .golangci.yml configuration

config/config_test.go

@@ -0,0 +1,32 @@
+package config
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestValidateDefaults(t *testing.T) {
+	cfg := WalConfig{}
+	err := cfg.Validate()
+	require.NoError(t, err, "default WalConfig should pass validation")
+
+	d := Defaults()
+	assert.Equal(t, d.MaxSegmentSize, cfg.MaxSegmentSize, "MaxSegmentSize should be defaulted")
+	assert.Equal(t, d.BlockSize, cfg.BlockSize, "BlockSize should be defaulted")
+	assert.Equal(t, d.SyncMode, cfg.SyncMode, "SyncMode should be defaulted")
+	assert.Equal(t, d.MaxBatchEntries, cfg.MaxBatchEntries, "MaxBatchEntries should be defaulted")
+	assert.Equal(t, d.MaxBatchSize, cfg.MaxBatchSize, "MaxBatchSize should be defaulted")
+	assert.Equal(t, d.MaxKeyBytes, cfg.MaxKeyBytes, "MaxKeyBytes should be defaulted")
+	assert.Equal(t, d.MaxInlineValue, cfg.MaxInlineValue, "MaxInlineValue should be defaulted")
+	assert.Equal(t, d.MemTableSize, cfg.MemTableSize, "MemTableSize should be defaulted")
+	assert.Equal(t, d.MaxImmutableCount, cfg.MaxImmutableCount, "MaxImmutableCount should be defaulted")
+}
+
+func TestValidateSegmentTooSmall(t *testing.T) {
+	cfg := WalConfig{MaxSegmentSize: 1024}
+	err := cfg.Validate()
+	require.Error(t, err, "MaxSegmentSize=1024 should fail validation")
+	assert.Contains(t, err.Error(), "too small")
+}