feat: three render modes — terminal, ascii, graphics

Three render modes:
- --mode terminal: pure ANSI ASCII in terminal
- --mode ascii: Vulkan window with ASCII characters (glyph atlas)
- --mode graphics: Vulkan window with colored cells (no glyphs, each
  material = unique base color, no lighting yet)

Graphics renderer:
- Same Vulkan pipeline as ASCII but without glyph atlas
- Simpler shaders: graphics.vert/frag just output instance color
- No descriptor set, no texture sampling
- Each cell = colored quad, material color fills entire cell
- Entities rendered as colored shapes (player=yellow, goblin=green)

Default mode changed to --mode ascii
109 tests, 0 failures

assets/shaders/graphics.frag

@@ -0,0 +1,9 @@
+#version 450
+
+layout(location = 0) in vec4 in_color;
+
+layout(location = 0) out vec4 out_color;
+
+void main() {
+    out_color = in_color;
+}

assets/shaders/graphics.vert

@@ -0,0 +1,22 @@
+#version 450
+
+layout(push_constant) uniform PC {
+    vec2 screen_size;
+    vec2 cell_size;
+} pc;
+
+layout(location = 0) in vec2 in_pos;
+layout(location = 1) in vec2 in_grid;
+layout(location = 2) in vec4 in_color;
+
+layout(location = 0) out vec4 out_color;
+
+void main() {
+    vec2 pixel = (in_grid + in_pos) * pc.cell_size;
+    gl_Position = vec4(
+        2.0 * pixel.x / pc.screen_size.x - 1.0,
+        2.0 * pixel.y / pc.screen_size.y - 1.0,
+        0.0, 1.0
+    );
+    out_color = in_color;
+}