skeleton: GtkApplication, window, GSettings schema, desktop, tests (ft0)

M0 (task ft0): the app shell — no feature views yet (those are M1 large, M7
grid).

- src/main.c: g_application_run + --version via handle-local-options (--help
  is GApplication's own); exits before activate so --version works headless.
- src/app.{c,h}: GgazeApp : AdwApplication : GtkApplication (libadwaita
  styling, decision #29), single-instance (decision #32),
  G_APPLICATION_HANDLES_OPEN; activate + open reuse the active window
  (replace-on-new-open); ggaze_window_open remembers the GFile.
- src/window.{c,h}: GgazeWindow : GtkApplicationWindow with AdwHeaderBar and a
  GtkStack (children 'grid'/'large', default 'grid'); dispose clears the file.
- data/org.buetow.ggaze.desktop (image/* MIME, validated) and
  data/org.buetow.ggaze.gschema.xml (all keys + defaults, --strict clean).
- meson: libggaze static lib shared by the executable and tests; install to
  bindir/applications/glib-2.0 schemas; gnome.post_install compiles schemas +
  updates desktop db.
- tests: unit test_app (--version/--help/unknown via subprocess), integration
  test_window (offscreen stack checks + open-titles), bootstrap. 3/3 green.
- docs/tech-stack.md: window-geometry shape corrected to (width,height,
  fullscreen,maximized) to match the schema.

1 files changed, 80 insertions, 0 deletions

diff --git a/tests/test_app.c b/tests/test_app.c
new file mode 100644
index 0000000..b1438a1
--- /dev/null
+++ b/tests/test_app.c
@@ -0,0 +1,80 @@
+/*:*
+ * ggaze — app CLI unit test
+ *
+ * Exercises the ggaze binary as a subprocess (so --help/--version, which call
+ * exit() inside g_application_run, do not terminate the test process itself).
+ * Verifies: `--version` prints "ggaze" and exits 0; `--help` exits 0; an
+ * unknown flag exits non-zero. No display is needed — all three exit before
+ * activate. The ggaze binary path is passed as argv[1] by meson.
+ *
+ * Copyright (c) 2026 ggaze contributors
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *:*/
+
+#include "ggaze-config.h"
+
+#include <glib.h>
+#include <gio/gio.h>
+
+static const char *GGAZE_BIN;
+
+/* Run GGAZE_BIN with c_arg, return the exit status (0 on success). If
+ * c_out is non-NULL, receive the combined stdout+stderr. */
+static gint
+run_one(const char *c_arg, char **c_out) {
+   GError      *p_err = NULL;
+   GSubprocess *p_sub = g_subprocess_new(G_SUBPROCESS_FLAGS_STDOUT_PIPE |
+                                            G_SUBPROCESS_FLAGS_STDERR_MERGE,
+                                         &p_err, GGAZE_BIN, c_arg, NULL);
+   g_assert_no_error(p_err);
+
+   char *c_stdout = NULL;
+   g_subprocess_communicate_utf8(p_sub, NULL, NULL, &c_stdout, NULL, &p_err);
+   g_assert_no_error(p_err);
+
+   g_assert_true(g_subprocess_get_if_exited(p_sub));
+   gint i_status = g_subprocess_get_exit_status(p_sub);
+
+   if (c_out != NULL) {
+      *c_out = c_stdout;
+   } else {
+      g_free(c_stdout);
+   }
+   g_object_unref(p_sub);
+   return (i_status);
+}
+
+static void
+test_version(void) {
+   char *c_out    = NULL;
+   gint  i_status = run_one("--version", &c_out);
+   g_assert_cmpint(i_status, ==, 0);
+   g_assert_nonnull(c_out);
+   g_assert_nonnull(g_strstr_len(c_out, -1, "ggaze"));
+   g_free(c_out);
+}
+
+static void
+test_help(void) {
+   /* GApplication provides --help itself; it prints and exits 0. */
+   gint i_status = run_one("--help", NULL);
+   g_assert_cmpint(i_status, ==, 0);
+}
+
+static void
+test_unknown_flag(void) {
+   gint i_status = run_one("--definitely-not-a-ggaze-flag", NULL);
+   g_assert_cmpint(i_status, !=, 0);
+}
+
+int
+main(int i_argc, char **c_argv) {
+   g_assert_cmpint(i_argc, >=, 2);
+   GGAZE_BIN = c_argv[1];
+
+   g_test_init(&i_argc, &c_argv, NULL);
+   g_test_add_func("/app/version", test_version);
+   g_test_add_func("/app/help", test_help);
+   g_test_add_func("/app/unknown_flag", test_unknown_flag);
+   return (g_test_run());
+}
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