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/vulkan/ra3.vulkan.null.cppm

@@ -0,0 +1,25 @@
+export module ra3.vulkan;
+
+import std;
+
+export import ra3.core;
+import ra3.render;
+import ra3.client;
+
+/**
+ * Fallback Vulkan backend used when the build has no Vulkan loader (for example
+ * the Windows cross-build without a Vulkan SDK). `init` fails so the caller can
+ * select another display.
+ */
+export namespace ra3::vulkan {
+    class vulkan_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 "vulkan(null)"; }
+    };
+}