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.

shaders/scene.frag

@@ -0,0 +1,11 @@
+#version 450
+
+layout(binding = 0) uniform sampler2D scene;
+
+layout(location = 0) in vec2 in_uv;
+layout(location = 0) out vec4 out_color;
+
+void main() {
+    if (in_uv.x < 0.0 || in_uv.x > 1.0 || in_uv.y < 0.0 || in_uv.y > 1.0) discard;
+    out_color = texture(scene, in_uv);
+}