Introduces watch/, a real Pebble watchapp (built via pebble build) that

shows the daily tarot/astrology reading on-device: persistence, reading
computation, and UI screens, driven by the existing engine/interpreter
code linked in unchanged where possible.

Required engine-side changes to make that linking work:
- A compact low-precision ephemeris (lowprec_ephemeris.c) replacing the
  vendored ~127KB Astronomy Engine, which doesn't fit the watch's ~64KB
  app budget.
- Hand-rolled sqrt/atan2/sin/cos replacements for that ephemeris and
  astro.c's Ascendant calculation - Pebble's statically-linked libm
  hard-faults on real hardware under this app's -fPIE link for all four.
- narrative.c/guidance.c refactored from FILE*/fprintf onto snprintf-
  based buffers, since Pebble's SDK blocks fprintf at compile time.

watch/src/c/main.c

@@ -0,0 +1,50 @@
+#include <pebble.h>
+
+#include "../../../engine/src/i18n.h"
+#include "app_message.h"
+#include "config.h"
+#include "i18n_tables.h"
+#include "reading_state.h"
+#include "ui_menu_window.h"
+#include "ui_text_window.h"
+
+static WatchConfig s_config;
+
+static void show_setup_required(void) {
+  text_window_push(i18n_get("ui.setup_required_title", "Setup Required"),
+                    i18n_get("ui.setup_required_body",
+                             "Open this app's settings from the Pebble mobile app to add your "
+                             "birth details, then reopen Deck in a Dash."));
+}
+
+static void on_config_updated(const WatchConfig *cfg) {
+  bool was_configured = s_config.configured;
+  s_config = *cfg;
+  watch_i18n_load(s_config.lang);
+
+  if (!s_config.configured) return;
+
+  reading_state_recompute(&s_config.birth);
+  if (was_configured) {
+    menu_window_reload();
+  } else {
+    menu_window_push();
+  }
+}
+
+int main(void) {
+  config_load(&s_config);
+  watch_i18n_load(s_config.lang);
+  app_message_init(on_config_updated);
+
+  if (s_config.configured) {
+    reading_state_recompute(&s_config.birth);
+    menu_window_push();
+  } else {
+    show_setup_required();
+  }
+
+  app_event_loop();
+
+  menu_window_deinit();
+}