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/lowprec_ephemeris.h

@@ -0,0 +1,40 @@
+#ifndef DECK_LOWPREC_EPHEMERIS_H
+#define DECK_LOWPREC_EPHEMERIS_H
+
+#include "astro.h"
+
+/* A compact, self-contained low-precision ephemeris for the watch build
+ * only (PBL_SDK_3) - the vendored Astronomy Engine (astro.c's normal
+ * desktop path) compiles to ~127KB of code, well over a whole Pebble
+ * app's entire 64KB code+data+bss budget, so it can't be linked into the
+ * watchapp at all. This module replaces it there via classic Keplerian
+ * orbital-element formulas (the widely-published "low precision"
+ * planetary position method - see lowprec_ephemeris.c's own comment for
+ * accuracy and sourcing). Not used, and not compiled, on desktop - see
+ * astro.c's AstroTime typedef and the Makefile's ENGINE_SRCS, which never
+ * includes this file. */
+
+/* Julian Date (TT and UT are treated as identical at this precision -
+ * their few-tens-of-seconds difference is far below this method's own
+ * ~1 arcminute-to-a-few-degrees accuracy) for the given UTC calendar
+ * date/time. */
+double lowprec_julian_date(int year, int month, int day, int hour, int minute, double second);
+
+/* Geocentric apparent ecliptic longitude of `body` at Julian Date `jd`,
+ * in degrees [0, 360). */
+double lowprec_geocentric_longitude(Body body, double jd);
+
+/* Greenwich Mean Sidereal Time at Julian Date `jd`, in hours [0, 24) -
+ * the watch's replacement for Astronomy_SiderealTime(), used the same
+ * way (RAMC = gmst*15 + geographic longitude) to compute the Ascendant. */
+double lowprec_gmst_hours(double jd);
+
+/* atan2()/sin()/cos() replacements for the watch build - see
+ * lowprec_ephemeris.c's file-level comment. Exported (not file-static)
+ * since astro.c's compute_ascendant() also needs them. Same signature/
+ * semantics as their libm counterparts, in radians. */
+double lowprec_atan2(double y, double x);
+double lowprec_sin(double rad);
+double lowprec_cos(double rad);
+
+#endif