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/narrative.c

@@ -136,8 +136,11 @@ static const char *narrative_field(const char *slug, const char *field, const ch
   return i18n_get(key, fallback);
 }
 
-void narrative_print(FILE *out, const char *indent, const Aspect *aspect,
+void narrative_write(char *buf, size_t buf_size, const char *indent, const Aspect *aspect,
                       int transiting_house) {
+  if (buf_size == 0) return;
+  buf[0] = '\0';
+
   const TransitNarrative *n = &k_narratives[aspect->transiting_planet];
   const char *base = narrative_field(n->slug, "base", n->base);
 
@@ -147,7 +150,15 @@ void narrative_print(FILE *out, const char *indent, const Aspect *aspect,
 
   if (base[0] == '\0' && extra[0] == '\0') return;
 
-  fprintf(out, "%s", indent);
+  /* Chained snprintf() calls, each bounds-checked against the running
+   * length before the next one runs - never fprintf/sprintf/vsnprintf,
+   * all of which Pebble's SDK blocks at compile time (see narrative.h's
+   * doc comment). Once `len` reaches buf_size (truncated), every
+   * subsequent call is skipped rather than computing buf_size - len,
+   * which would underflow. */
+  size_t len = (size_t)snprintf(buf, buf_size, "%s", indent);
+  if (len >= buf_size) return;
+
   if (transiting_house >= 1 && transiting_house <= 12) {
     char house_key[24];
     snprintf(house_key, sizeof house_key, "narrative.house.%d", transiting_house);
@@ -156,9 +167,16 @@ void narrative_print(FILE *out, const char *indent, const Aspect *aspect,
     char frame[256];
     snprintf(frame, sizeof frame, i18n_get("narrative.house_frame", "In matters of %s (house %d)."),
              area, transiting_house);
-    fprintf(out, "%s ", frame);
+    len += (size_t)snprintf(buf + len, buf_size - len, "%s ", frame);
+    if (len >= buf_size) return;
   }
-  if (base[0] != '\0') fprintf(out, "%s", base);
-  if (extra[0] != '\0') fprintf(out, "%s%s", base[0] != '\0' ? " " : "", extra);
-  fprintf(out, "\n");
+  if (base[0] != '\0') {
+    len += (size_t)snprintf(buf + len, buf_size - len, "%s", base);
+    if (len >= buf_size) return;
+  }
+  if (extra[0] != '\0') {
+    len += (size_t)snprintf(buf + len, buf_size - len, "%s%s", base[0] != '\0' ? " " : "", extra);
+    if (len >= buf_size) return;
+  }
+  snprintf(buf + len, buf_size - len, "\n");
 }