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.

engine/src/reading.c

@@ -3,13 +3,6 @@
 
 #include <math.h>
 
-/* UI-label keys ("ui.*") are looked up here rather than in i18n.h/.c
- * because they're specific to these two renderers, not part of the
- * portable astro/tarot vocabulary. */
-static const char *ui(const char *key, const char *fallback) {
-  return i18n_get(key, fallback);
-}
-
 void reading_generate(const char *tarot_seed, const BirthData *birth,
                        time_t utc_moment, DailyReading *out) {
   astro_compute_natal_chart(birth, &out->natal);
@@ -17,6 +10,18 @@ void reading_generate(const char *tarot_seed, const BirthData *birth,
   tarot_draw_celtic_cross(tarot_seed, &out->spread);
 }
 
+/* Everything below uses fprintf, which Pebble's SDK blocks at compile
+ * time (see pebble_warn_unsupported_functions.h) - not compiled at all
+ * for the watch, which only ever calls reading_generate() above. */
+#ifndef PBL_SDK_3
+
+/* UI-label keys ("ui.*") are looked up here rather than in i18n.h/.c
+ * because they're specific to these two renderers, not part of the
+ * portable astro/tarot vocabulary. */
+static const char *ui(const char *key, const char *fallback) {
+  return i18n_get(key, fallback);
+}
+
 static void print_position_deg(FILE *out, const PlanetPosition *p) {
   fprintf(out, "%5.1f%s %-11s (%s %d)", p->degree_in_sign, "\xc2\xb0",
           astro_sign_name(p->sign), ui("ui.house", "House"), p->house);
@@ -194,3 +199,5 @@ void reading_print_json(const DailyReading *r, FILE *out) {
   }
   fprintf(out, "    ]\n  }\n}\n");
 }
+
+#endif /* !PBL_SDK_3 */