fullscreen: info/EXIF, fullscreen, slideshow, Esc back (ot0)

M4 (task ot0): fullscreen + slideshow + info overlay.

- src/info.{c,h}: plain-C EXIF gather via libexif (camera, lens, focal,
  aperture, shutter, ISO, datetime, orientation) + file info (dims, format,
  size). info_format() produces the overlay text. Unit-tested.
- src/window.c: fullscreen toggle (f), slideshow (S, 3s auto-advance), info
  overlay (i, GtkLabel in GtkOverlay, auto-hide 5s), Esc contextual back
  (fullscreen→unfullscreen→large→grid→quit). Stack wrapped in GtkOverlay.
  ggaze_window_get_stack() accessor for tests.
- src/shortcuts.c: f, S, i, Escape bindings.
- meson.build: libexif dep added.
- tests: unit test_info (EXIF extraction + orientation tag). 14/14 green,
  ASan clean.

1 files changed, 45 insertions, 0 deletions

diff --git a/src/info.h b/src/info.h
new file mode 100644
index 0000000..7680b72
--- /dev/null
+++ b/src/info.h
@@ -0,0 +1,45 @@
+#ifndef GGAZE_INFO_H
+#define GGAZE_INFO_H
+
+/*:*
+ * ggaze — image info / EXIF gather
+ *
+ * Gathers file info (dimensions, format, size) + EXIF (camera, lens, focal,
+ * aperture, shutter, ISO, datetime, orientation) into a GgazeInfo struct.
+ * Plain-C, no GtkWidget; uses GdkPixbuf (dims) + libexif (EXIF). Unit-testable.
+ *
+ * Copyright (c) 2026 ggaze contributors
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *:*/
+
+#include <gio/gio.h>
+#include <glib.h>
+
+G_BEGIN_DECLS
+
+typedef struct {
+   int    i_width;
+   int    i_height;
+   char  *c_format;      /* content type (owned) */
+   gint64 i_size;        /* file size in bytes */
+   char  *c_camera;      /* Make + Model (owned) */
+   char  *c_lens;        /* Lens model (owned, or NULL) */
+   char  *c_focal;       /* FocalLength (owned) */
+   char  *c_aperture;    /* FNumber (owned) */
+   char  *c_shutter;     /* ExposureTime (owned) */
+   char  *c_iso;         /* ISOSpeedRatings (owned) */
+   char  *c_datetime;    /* DateTimeOriginal (owned) */
+   int    i_orientation; /* EXIF Orientation (1-8, 0 if none) */
+} GgazeInfo;
+
+/* Gather info for p_file (synchronous). Returns a new GgazeInfo (caller owns,
+ * free with info_delete) or NULL on failure. */
+GgazeInfo *info_new(GFile *p_file);
+void       info_delete(GgazeInfo *p_info);
+
+/* Format the info as a multi-line string (caller frees). */
+char *info_format(const GgazeInfo *p_info);
+
+G_END_DECLS
+
+#endif /* GGAZE_INFO_H */
\ No newline at end of file