feat(12-01): report the search engine's found count and field weights

- optional beachcomber.PageSearcher returns a page of candidate ids plus
  the engine's found count; beachcomber.SearchPage falls back to
  SearchIDs for engines without it, so Engine is unchanged
- Query.QueryByWeights is sent to Typesense as query_by_weights; a
  mismatched weight list or a page above typesense.MaxPerPage (250) is
  refused before any request

modules/beachcomber/engines.go

@@ -62,7 +62,7 @@ func init() {
 }
 
 // nullEngine is never configured, so sync is skipped before it is called.
-// Every method is a no-op; SearchIDs returns an empty list.
+// Every method is a no-op; SearchIDs and SearchPage return no ids.
 type nullEngine struct{}
 
 func (nullEngine) Name() string     { return NullEngine }
@@ -75,3 +75,6 @@ func (nullEngine) Flush(context.Context, string) error            { return nil }
 func (nullEngine) SearchIDs(context.Context, string, Query) ([]string, error) {
 	return []string{}, nil
 }
+func (nullEngine) SearchPage(context.Context, string, Query) (SearchResult, error) {
+	return SearchResult{IDs: []string{}}, nil
+}