refactor: split dbx package into domain files

Reorganize monolithic dbx.go and utility.go into focused files by
concern, add package doc.go, and gitignore coverage.out.

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

jet_expr.go

@@ -0,0 +1,26 @@
+package dbx
+
+import (
+	"fmt"
+
+	"github.com/go-jet/jet/v2/mysql"
+)
+
+// ExprValues converts a list of values to a list of mysql.Expression values using
+// function f to transform the values (mysql.String for strings, mysql.Uint64, etc).
+func ExprValues[T any](values []T, f func(T) mysql.Expression) []mysql.Expression {
+	expressions := make([]mysql.Expression, len(values))
+	for i, v := range values {
+		expressions[i] = f(v)
+	}
+	return expressions
+}
+
+// ExprStringers converts a list of fmt.Stringers to a list of mysql.Expression values.
+func ExprStringers(values []fmt.Stringer) []mysql.Expression {
+	expressions := make([]mysql.Expression, len(values))
+	for i, v := range values {
+		expressions[i] = mysql.String(v.String())
+	}
+	return expressions
+}