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.

watch/src/c/ui_text_window.h

@@ -0,0 +1,16 @@
+#ifndef WATCH_UI_TEXT_WINDOW_H
+#define WATCH_UI_TEXT_WINDOW_H
+
+/* Pushes a scrollable full-screen text window: `title` as a bold header,
+ * `body` below it, word-wrapped and scrollable with the up/down buttons.
+ * Used for every screen that's just prose (Summary/Guidance, a single
+ * transit's narrative) - the Celtic Cross card screens use
+ * ui_card_window.h instead, which also shows the card's image.
+ *
+ * Copies both strings internally (into the pushed window's own heap
+ * state, freed when it's popped), so the caller's buffer doesn't need to
+ * outlive the call - every caller so far builds `title`/`body` in a
+ * short-lived local buffer right before pushing. */
+void text_window_push(const char *title, const char *body);
+
+#endif