v0.3.0: display abstraction, map-browser menu, SAGE terrain tiling/blends

- ra3.client::display: shared interactive loops; SDL and Vulkan backends
  implement only the primitives (init/present/poll_event/window_size/
  key_down/present_terrain). ra3.display picks the backend.
- Menu: maps by localized name (gamestrings.csf), red/gold theme, hover
  highlight, mouse + keyboard, wheel scroll, fullscreen and FPS/vsync
  options, loading progress bar.
- Terrain: continuous tile sampling via a texture array (REPEAT, uv =
  cell/(2*cellSize)) removes per-cell grid seams; SAGE blend ramp for
  material transitions; FPS label + top-right minimap overlays.
- Skip the skirmish sim for map views; reuse the Vulkan texture; no idle
  terrain redraw.

src/ui/ra3.ui.null.cppm

@@ -3,19 +3,21 @@ export module ra3.ui;
 import std;
 export import ra3.core;
 import ra3.render;
+import ra3.client;
 
 /**
- * Fallback viewer used when the build has no SDL3. It never opens a window;
- * the caller should write an offscreen image instead.
+ * Fallback SDL backend used when the build has no SDL3. It never opens a window;
+ * `init` fails so the caller can fall back to an offscreen image.
  */
 export namespace ra3::ui {
-    /** Window parameters (mirrors the SDL backend). */
-    struct viewer_options {
-        std::string title = "OpenRA3";
-        int width = 1024;
-        int height = 768;
+    class sdl_display final : public ra3::client::display {
+    public:
+        [[nodiscard]] auto init(const ra3::client::display_options &) -> bool override { return false; }
+        [[nodiscard]] auto present(const ra3::render::image &, const ra3::render::view_rect &, bool) -> bool override { return false; }
+        [[nodiscard]] auto poll_event(ra3::render::ui_event &) -> bool override { return false; }
+        [[nodiscard]] auto window_size() const -> std::pair<int, int> override { return {0, 0}; }
+        [[nodiscard]] auto key_down(ra3::render::ui_key) const -> bool override { return false; }
+        auto shutdown() -> void override {}
+        [[nodiscard]] auto name() const -> std::string_view override { return "sdl(null)"; }
     };
-
-    /** Always returns false: this build has no windowing backend. */
-    [[nodiscard]] inline auto run_viewer(const ra3::render::image &, const viewer_options &) -> bool { return false; }
 }