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.go

@@ -0,0 +1,44 @@
+package dbx
+
+import (
+	"fmt"
+
+	"github.com/go-jet/jet/v2/mysql"
+)
+
+// CountResult holds the count from a Jet COUNT query. Use with a SELECT that
+// aliases the count as `CountResult.Count`, for example:
+//
+//	SELECT(mysql.COUNT(col).AS("CountResult.Count"))
+type CountResult struct {
+	Count int
+}
+
+// QueryCountFn counts rows matching pre-bound table and condition parameters.
+type QueryCountFn func(sqlo Queryable) (int, error)
+
+// QueryCount counts rows in tbl matching conds.
+func QueryCount(
+	sqlo Queryable,
+	col Column,
+	tbl ReadableTable,
+	conds BoolExpression,
+) (int, error) {
+	stmt := tbl.SELECT(mysql.COUNT(col).AS("CountResult.Count")).WHERE(conds)
+	var res CountResult
+	if err := stmt.Query(sqlo, &res); err != nil {
+		return 0, fmt.Errorf("query count: %w", err)
+	}
+	return res.Count, nil
+}
+
+// BuildQueryCountFn returns a QueryCountFn with col, tbl, and conds bound.
+func BuildQueryCountFn(
+	col Column,
+	tbl ReadableTable,
+	conds BoolExpression,
+) QueryCountFn {
+	return func(sqlo Queryable) (int, error) {
+		return QueryCount(sqlo, col, tbl, conds)
+	}
+}