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.

Dockerfile

@@ -34,6 +34,7 @@ RUN --mount=type=cache,target=/var/lib/apt,sharing=locked \
         libc++-21-dev \
         libc++abi-21-dev \
         libsdl3-dev \
+        libvulkan-dev \
         ninja-build \
         pkg-config \
     && rm -rf /var/lib/apt/lists/*
@@ -56,10 +57,11 @@ ENV DEBIAN_FRONTEND=noninteractive
 RUN --mount=type=cache,target=/var/lib/apt,sharing=locked \
     --mount=type=cache,target=/var/cache/apt,sharing=locked \
     apt-get -o Acquire::Retries=5 update \
-    && apt-get -o Acquire::Retries=5 install -y --no-install-recommends \
+    &&     apt-get -o Acquire::Retries=5 install -y --no-install-recommends \
         libc++1 \
         libc++abi1 \
         libsdl3-0 \
+        libvulkan1 \
     && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app