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.

.gitignore

@@ -2,6 +2,9 @@
 # data once filled in — never commit it.
 /dist
 /build
+/watch/build
+/watch/src/c/i18n_tables.auto.c
+/watch/resources/img
 
 # Durable backup of the same real birth data, outside dist/ so a dist/
 # wipe doesn't lose it — see the Makefile's `scripts` target. Never