feat: add delete, context, and tx helpers

Add Delete/DeleteAffected, context-aware CRUD helpers, WithTx, ContainsCol,
and queryReturning deduplication for returning statements.

Co-authored-by: Cursor <cursoragent@cursor.com>

query_test.go

@@ -0,0 +1,80 @@
+package dbx
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestFetch_EmptyOnNoRows(t *testing.T) {
+	stmt := mockStatement{
+		queryContextFn: func(_ context.Context, dest any) error {
+			return ErrNoRows
+		},
+	}
+
+	result, err := Fetch[row](mockQueryable{}, stmt)
+	require.NoError(t, err)
+	assert.Empty(t, result)
+}
+
+func TestFetch_ReturnsRows(t *testing.T) {
+	stmt := mockStatement{
+		queryContextFn: func(_ context.Context, dest any) error {
+			ptr := dest.(*[]*row)
+			*ptr = []*row{{ID: 1}, {ID: 2}}
+			return nil
+		},
+	}
+
+	result, err := Fetch[row](mockQueryable{}, stmt)
+	require.NoError(t, err)
+	assert.Len(t, result, 2)
+}
+
+func TestMustFetch_NotFound(t *testing.T) {
+	errNotFound := errors.New("not found")
+	stmt := mockStatement{
+		queryContextFn: func(_ context.Context, _ any) error {
+			return ErrNoRows
+		},
+	}
+
+	result, err := MustFetch[row](mockQueryable{}, stmt, errNotFound)
+	assert.Nil(t, result)
+	assert.ErrorIs(t, err, errNotFound)
+}
+
+func TestFetchContext_PropagatesContext(t *testing.T) {
+	ctx := context.WithValue(context.Background(), testContextKey{}, "ok")
+	var gotCtx context.Context
+	stmt := mockStatement{
+		queryContextFn: func(c context.Context, dest any) error {
+			gotCtx = c
+			ptr := dest.(*[]*row)
+			*ptr = []*row{{ID: 3}}
+			return nil
+		},
+	}
+
+	_, err := FetchContext[row](ctx, mockQueryable{}, stmt)
+	require.NoError(t, err)
+	assert.Equal(t, ctx, gotCtx)
+}
+
+type testContextKey struct{}
+
+func TestFetchOne_NilWhenEmpty(t *testing.T) {
+	stmt := mockStatement{
+		queryContextFn: func(_ context.Context, _ any) error {
+			return ErrNoRows
+		},
+	}
+
+	result, err := FetchOne[row](mockQueryable{}, stmt)
+	require.NoError(t, err)
+	assert.Nil(t, result)
+}