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

errors_test.go

@@ -0,0 +1,42 @@
+package go_kv
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func TestErrorTypes(t *testing.T) {
+	allErrors := []error{
+		ErrCommitUnknown,
+		ErrWriteStopped,
+		ErrSequenceExhausted,
+		ErrWALCorrupted,
+		ErrInvalidConfig,
+	}
+
+	// Each error must match itself via errors.Is.
+	for _, err := range allErrors {
+		if !errors.Is(err, err) {
+			t.Errorf("errors.Is(%v, %v) = false, want true", err, err)
+		}
+	}
+
+	// Each error must NOT match any other error.
+	for i, a := range allErrors {
+		for j, b := range allErrors {
+			if i == j {
+				continue
+			}
+			if errors.Is(a, b) {
+				t.Errorf("errors.Is(%v, %v) = true, want false (distinct errors)", a, b)
+			}
+		}
+	}
+
+	// Wrapped errors must still be identifiable via errors.Is.
+	wrapped := fmt.Errorf("operation failed: %w", ErrCommitUnknown)
+	if !errors.Is(wrapped, ErrCommitUnknown) {
+		t.Errorf("errors.Is(wrapped ErrCommitUnknown, ErrCommitUnknown) = false, want true")
+	}
+}