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/astro.h

@@ -1,7 +1,17 @@
 #ifndef DECK_ASTRO_H
 #define DECK_ASTRO_H
 
+/* Pebble's SDK pre-defines <time.h>'s own include guard (so a real
+ * #include <time.h> is a silent no-op) and instead declares struct
+ * tm/time_t/gmtime()/etc. itself, inside pebble.h - see PBL_SDK_3,
+ * predefined by the Pebble build for every target platform. Desktop
+ * builds (deck-engine, the interpreter, and this file's own tests) use
+ * plain <time.h> as normal. */
+#ifdef PBL_SDK_3
+#include <pebble.h>
+#else
 #include <time.h>
+#endif
 
 #define NUM_BODIES 10
 #define MAX_ASPECTS 100