feat: xorg text overlays, font atlas generator, v4l2_view_cli

- tools/gen_font_atlas: Python/Pillow build tool — skyline packs DejaVu
  Sans glyphs 32-255 into a grayscale atlas, emits build/gen/font_atlas.h
  with pixel data and Font_Glyph[256] metrics table
- xorg: bitmap font atlas text overlay rendering (GL_R8 atlas texture,
  alpha-blended glyph quads, dark background rect per overlay)
- xorg: add xorg_viewer_set_overlay_text / clear_overlays API
- xorg: add xorg_viewer_handle_events for streaming use (events only,
  no redundant render)
- xorg_cli: show today's date as white text overlay
- v4l2_view_cli: new tool — V4L2 capture with format auto-selection
  (highest FPS then largest resolution), MJPEG/YUYV, measured FPS overlay
- docs: update README, planning, architecture to reflect current status

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

dev/cli/xorg_cli.c

@@ -1,6 +1,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <time.h>
 
 #include "test_image.h"
 #include "xorg.h"
@@ -100,14 +101,24 @@ int main(int argc, char **argv)
 	       width, height, fmt_name, pat_name, scale_name, anchor_name, win_x, win_y);
 
 	Xorg_Viewer *v = xorg_viewer_open(win_x, win_y, width, height, "xorg_cli");
-	xorg_viewer_set_scale(v, scale);
-	xorg_viewer_set_anchor(v, anchor);
 	if (!v) {
 		fprintf(stderr, "xorg_cli: failed to open viewer window\n");
 		test_image_free(f);
 		return 1;
 	}
 
+	xorg_viewer_set_scale(v, scale);
+	xorg_viewer_set_anchor(v, anchor);
+
+	/* Overlay: today's date, white text, top-left corner. */
+	{
+		char date_buf[32];
+		time_t     now = time(NULL);
+		struct tm *tm  = localtime(&now);
+		strftime(date_buf, sizeof(date_buf), "%Y-%m-%d", tm);
+		xorg_viewer_set_overlay_text(v, 0, 10, 10, date_buf, 1.0f, 1.0f, 1.0f);
+	}
+
 	if (fmt == TEST_FMT_YUV420) {
 		xorg_viewer_push_yuv420(v,
 		    f->plane[0], f->plane[1], f->plane[2],