feat(lagoon): per-query collation for OrderBy, drop the database locale check

- lagoon.OrderBy takes variadic lagoon.OrderOption values; lagoon.Collate(name)
  emits a validated, double-quoted COLLATE clause (e.g. "pl-x-icu")
- remove the exported CheckLocale and the ICU pl-PL check from Open and Use
- framework test containers and per-test databases are plain PostgreSQL
- lagoon README, root README and docs pages drop the locale requirement;
  queries-and-pagination gains a "Sorting with a collation" section
  backed by ExampleCollate

modules/lagoon/example_test.go

@@ -208,6 +208,27 @@ func ExampleOrderBy() {
 	// lagoon: order direction "asc; DROP TABLE acme_blog_posts" is not allow-listed
 }
 
+func ExampleCollate() {
+	// A dry-run handle shows the SQL without a database.
+	db, _ := gorm.Open(postgres.New(postgres.Config{DSN: "host=127.0.0.1"}), &gorm.Config{DryRun: true, DisableAutomaticPing: true})
+	allowed := []string{"title", "views"}
+
+	// Polish alphabetical order, whatever the database's default locale.
+	q, err := lagoon.OrderBy(db.Model(&Post{}), "title", "asc", allowed, lagoon.Collate("pl-x-icu"))
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	var posts []Post
+	fmt.Println(q.Find(&posts).Statement.SQL.String())
+
+	_, err = lagoon.OrderBy(db, "title", "asc", allowed, lagoon.Collate(`pl-x-icu" ASC, (SELECT 1) --`))
+	fmt.Println(err)
+	// Output:
+	// SELECT * FROM "acme_blog_posts" WHERE "acme_blog_posts"."deleted_at" IS NULL ORDER BY title COLLATE "pl-x-icu" ASC
+	// lagoon: order collation "pl-x-icu\" ASC, (SELECT 1) --" is not a valid collation name
+}
+
 func ExamplePaginate() {
 	rows := []map[string]any{{"id": 3, "title": "Third"}}
 	page := lagoon.Paginate(rows, 2, 2, 3)