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/ra3.cppm

@@ -9,9 +9,13 @@ export module ra3;
 export import ra3.core;
 export import ra3.logic;
 export import ra3.client;
+export import ra3.data;
 export import ra3.game;
 export import ra3.fs;
 export import ra3.map;
 export import ra3.skirmish;
 export import ra3.render;
+export import ra3.terrain;
 export import ra3.ui;
+export import ra3.vulkan;
+export import ra3.display;