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>

include/xorg.h

@@ -56,11 +56,27 @@ bool xorg_viewer_push_mjpeg(Xorg_Viewer *v,
 void xorg_viewer_set_scale(Xorg_Viewer *v, Xorg_Scale scale);
 void xorg_viewer_set_anchor(Xorg_Viewer *v, Xorg_Anchor anchor);
 
+/*
+ * Set a text overlay at window-pixel position (x, y) — top-left of the text.
+ * Up to 8 overlays (idx 0..7); calling with the same idx replaces it.
+ * r, g, b are in [0, 1].
+ */
+void xorg_viewer_set_overlay_text(Xorg_Viewer *v, int idx, int x, int y,
+                                   const char *text, float r, float g, float b);
+
+/* Remove all text overlays. */
+void xorg_viewer_clear_overlays(Xorg_Viewer *v);
+
 /*
  * Process pending window events.
  * Returns false when the user has closed the window.
  * Must be called from the thread that created the viewer.
+ *
+ * poll()           — processes events then re-renders; use for static images.
+ * handle_events()  — processes events only, no render; use when push_* drives
+ *                    rendering (e.g. live camera feed).
  */
 bool xorg_viewer_poll(Xorg_Viewer *v);
+bool xorg_viewer_handle_events(Xorg_Viewer *v);
 
 void xorg_viewer_close(Xorg_Viewer *v);