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/terrain.vert

@@ -0,0 +1,11 @@
+#version 450
+
+// Fullscreen triangle; the terrain is ray-marched in the fragment shader
+// (GPU), so the vertex stage only emits the screen UV.
+layout(location = 0) out vec2 out_uv;
+
+void main() {
+    vec2 p = vec2((gl_VertexIndex << 1) & 2, gl_VertexIndex & 2);
+    out_uv = p;
+    gl_Position = vec4(p * 2.0 - 1.0, 0.0, 1.0);
+}