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/core/ra3.core.cppm

@@ -11,8 +11,8 @@ import std;
  */
 export namespace ra3::core {
     inline constexpr int version_major = 0;
-    inline constexpr int version_minor = 2;
-    inline constexpr int version_patch = 1;
+    inline constexpr int version_minor = 3;
+    inline constexpr int version_patch = 0;
 
     using real = float;
     using int32 = std::int32_t;