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.

interpreter/src/guidance.h

@@ -1,7 +1,7 @@
 #ifndef DECK_GUIDANCE_H
 #define DECK_GUIDANCE_H
 
-#include <stdio.h>
+#include <stddef.h>
 
 /* Header-only dependency on the engine for struct/enum *definitions*,
  * same policy as narrative.h/significance.h - see their comments for
@@ -14,6 +14,13 @@
 #include "../../engine/src/tarot.h"
 #include "significance.h"
 
+/* Generous upper bound on guidance_write()'s output (intro/stance line
+ * + Attitude line + Outcome line, each ending in a newline, including
+ * the longest tarot_card_meaning() strings) across every language this
+ * project ships a translation for - see narrative.h's NARRATIVE_TEXT_MAX
+ * for the same reasoning. */
+#define GUIDANCE_TEXT_MAX 1024
+
 /* Ties the day's single most significant transit
  * (interp->top_items[0]) to the Celtic Cross spread, and prints a short
  * paragraph of concrete guidance on how to meet the day - the "combined
@@ -49,12 +56,18 @@
  * strings) is looked up via i18n_get() under "guidance.*" keys
  * (engine/i18n's .lang files) before falling back to the English text baked
  * into guidance.c, so it respects whatever --lang the caller loaded
- * with i18n_load() (main.c does so before calling this). The German
- * guidance.intro.* templates are deliberately phrased so the planet
- * name placeholder is always the sentence's grammatical subject
- * (nominative case) across all four day levels - see guidance.c's own
- * comment on k_intro_fallback. */
-void guidance_print(FILE *out, const char *indent, const DailyInterpretation *interp,
+ * with i18n_load()/i18n_load_table() (main.c does so before calling
+ * this). The German guidance.intro.* templates are deliberately phrased
+ * so the planet name placeholder is always the sentence's grammatical
+ * subject (nominative case) across all four day levels - see
+ * guidance.c's own comment on k_intro_fallback.
+ *
+ * Writes into `buf` (a caller-supplied buffer of `buf_size` bytes,
+ * always left null-terminated - GUIDANCE_TEXT_MAX is a safe size)
+ * instead of a FILE*, using only snprintf() internally (never
+ * fprintf/sprintf/vsnprintf, all of which Pebble's SDK blocks at compile
+ * time), so this function links into the watch app unchanged. */
+void guidance_write(char *buf, size_t buf_size, const char *indent, const DailyInterpretation *interp,
                      const CelticCrossSpread *spread);
 
 #endif