feat(count): add QueryCount pagination helpers

Adds CountResult, QueryCount, BuildQueryCountFn, and ReadableTable
alias for Jet pagination total counts, with tests and docs.

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

count_test.go

@@ -0,0 +1,24 @@
+package dbx
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestCountResult_Scan(t *testing.T) {
+	stmt := mockStatement{
+		queryContextFn: func(_ context.Context, dest any) error {
+			ptr := dest.(*CountResult)
+			ptr.Count = 3
+			return nil
+		},
+	}
+
+	var scanned CountResult
+	err := stmt.Query(mockQueryable{}, &scanned)
+	require.NoError(t, err)
+	assert.Equal(t, 3, scanned.Count)
+}