feat: add test_image module, xorg viewer sink, and feature flag build system

Feature flags (FEATURES= make variable):
  glfw       — GLFW + OpenGL viewer (libglfw3, libglew)
  vulkan     — future Vulkan renderer
  turbojpeg  — MJPEG decode/encode (libturbojpeg)
  xorg       — XRandR geometry + screen grab (libx11, libxrandr)
  vaapi      — VA-API hardware codec (libva)
Each flag injects -DHAVE_<FEATURE> and the relevant pkg-config flags.
Headless build: make (no FEATURES set).

test_image module (src/modules/test_image/):
  Generates test frames in YUV420, YUV422, and BGRA.
  Patterns: SMPTE 75% colour bars, greyscale ramp, white/black grid.
  BT.601 limited-range RGB→YCbCr in write_pixel().

test_image_cli (dev/cli/):
  Generates a frame and writes it as a PPM file for visual verification.
  Usage: test_image_cli [--pattern bars|ramp|grid]
                        [--width N] [--height N]
                        [--format yuv420|yuv422|bgra]
                        --out FILE.ppm

xorg module (src/modules/xorg/):
  xorg.c      — full GLFW+OpenGL implementation (compiled with FEATURES=glfw)
  xorg_stub.c — no-op stub (compiled otherwise; xorg_available() returns false)
  Renderer: full-screen quad via gl_VertexID, three GL_R8 textures for YUV,
  BT.601 matrix in fragment shader, GL_BGRA texture for packed frames.
  MJPEG path: tjDecompressToYUVPlanes → planar YUV → upload (requires turbojpeg).
  push_yuv420/push_bgra/push_mjpeg all usable independently.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/modules/xorg/xorg_stub.c

@@ -0,0 +1,36 @@
+#include <stddef.h>
+#include "xorg.h"
+
+bool xorg_available(void) { return false; }
+
+Xorg_Viewer *xorg_viewer_open(int x, int y, int width, int height,
+                               const char *title)
+{
+	(void)x; (void)y; (void)width; (void)height; (void)title;
+	return NULL;
+}
+
+bool xorg_viewer_push_yuv420(Xorg_Viewer *v,
+                              const uint8_t *y, const uint8_t *cb, const uint8_t *cr,
+                              int width, int height)
+{
+	(void)v; (void)y; (void)cb; (void)cr; (void)width; (void)height;
+	return false;
+}
+
+bool xorg_viewer_push_bgra(Xorg_Viewer *v,
+                            const uint8_t *data, int width, int height)
+{
+	(void)v; (void)data; (void)width; (void)height;
+	return false;
+}
+
+bool xorg_viewer_push_mjpeg(Xorg_Viewer *v,
+                             const uint8_t *data, size_t size)
+{
+	(void)v; (void)data; (void)size;
+	return false;
+}
+
+bool xorg_viewer_poll(Xorg_Viewer *v)  { (void)v; return false; }
+void xorg_viewer_close(Xorg_Viewer *v) { (void)v; }