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.h

@@ -1,8 +1,15 @@
 #ifndef DECK_READING_H
 #define DECK_READING_H
 
+/* See astro.h's own comment on this same #ifdef - Pebble's SDK
+ * pre-defines <time.h>'s include guard and declares time_t/struct tm
+ * itself inside pebble.h. */
+#ifdef PBL_SDK_3
+#include <pebble.h>
+#else
 #include <stdio.h>
 #include <time.h>
+#endif
 
 #include "astro.h"
 #include "tarot.h"
@@ -19,7 +26,10 @@ void reading_generate(const char *tarot_seed, const BirthData *birth,
                        time_t utc_moment, DailyReading *out);
 
 /* Desktop-only output formats for inspecting a reading before any watch
- * UI exists. Not used by (and not needed by) the watchapp. */
+ * UI exists - use fprintf, which Pebble's SDK blocks at compile time, so
+ * these aren't even declared (and reading.c doesn't define them) when
+ * building for the watch. Not used by (and not needed by) the watchapp. */
+#ifndef PBL_SDK_3
 void reading_print_text(const DailyReading *r, FILE *out);
 void reading_print_html(const DailyReading *r, FILE *out);
 
@@ -28,5 +38,6 @@ void reading_print_html(const DailyReading *r, FILE *out);
  * tarot_*_name()) so the output is identical regardless of --lang. This
  * is the interchange format interpreter/ reads - see its json.c. */
 void reading_print_json(const DailyReading *r, FILE *out);
+#endif
 
 #endif