Add worldwide genre style selector

docs/PRODUCT.md

@@ -21,6 +21,8 @@ and writing engine, then prepares final lyrics and style instructions.
 ## Song Controls
 
 -   Multiple genres/styles can be blended.
+-   Genres and styles can be selected from a large worldwide catalog with
+    search, category/region browsing and checkbox-based multi-select.
 -   Instruments can be selected from a large worldwide instrument
     catalog.
 -   Instruments can be searched by name, browsed by family/category and
@@ -75,6 +77,14 @@ Instrument images, audio previews, sound samples, audio playback,
 Freesound, an Instrument API and external API calls for instrument data
 are not part of the current product requirement.
 
+## Genre & Style Selector
+
+The application provides a large, curated local catalog of worldwide
+music genres and styles. Users can search it, browse by category and
+region/origin where useful, and select or deselect multiple entries with
+checkboxes. Catalog data is structured and separate from SwiftUI views;
+it does not require an external API.
+
 ## Project Management
 
 Every song is stored as a project. Important changes can create versions