feat(bookstack-api-m6n): implement pagination iterator with iter.Seq2

Add generic listAll helper using Go 1.23+ iter.Seq2 for memory-efficient
pagination. Implement ListAll() on BooksService and PagesService.
Tests cover multi-page iteration, early break, errors, and empty results.

iterator.go

@@ -0,0 +1,50 @@
+package bookstack
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"iter"
+)
+
+const defaultPageSize = 100
+
+// listAllResponse is used internally to get both data and total from paginated endpoints.
+type listAllResponse struct {
+	Data  json.RawMessage `json:"data"`
+	Total int             `json:"total"`
+}
+
+// listAll returns an iterator that paginates through all results for the given path.
+func listAll[T any](ctx context.Context, c *Client, path string) iter.Seq2[T, error] {
+	return func(yield func(T, error) bool) {
+		offset := 0
+		for {
+			var resp listAllResponse
+			url := fmt.Sprintf("%s?count=%d&offset=%d", path, defaultPageSize, offset)
+			if err := c.do(ctx, "GET", url, nil, &resp); err != nil {
+				var zero T
+				yield(zero, err)
+				return
+			}
+
+			var items []T
+			if err := json.Unmarshal(resp.Data, &items); err != nil {
+				var zero T
+				yield(zero, fmt.Errorf("unmarshaling page data: %w", err))
+				return
+			}
+
+			for _, item := range items {
+				if !yield(item, nil) {
+					return
+				}
+			}
+
+			offset += len(items)
+			if offset >= resp.Total || len(items) == 0 {
+				return
+			}
+		}
+	}
+}