Add JSON output format and a separate interpreter binary

deck-engine gains --format json (language-independent, slug-based)
so a new dist/interpreter-cli can score how significant a day's
transits are without linking the engine itself — it depends only on
the JSON shape, via its own minimal parser. Also documents the full
reading pipeline end to end (CLAUDE.md, README, docs/) and adds
docs/reading.md, a non-technical explanation of what a daily reading
contains and means.

engine/src/astro.c

@@ -231,3 +231,8 @@ const char *astro_moon_phase_name(MoonPhaseName phase) {
 const char *astro_aspect_name(AspectType type) {
   return lookup("aspect.", k_aspect_slug[type], k_aspect_names[type]);
 }
+
+const char *astro_body_slug(Body body) { return k_body_slug[body]; }
+const char *astro_sign_slug(ZodiacSign sign) { return k_sign_slug[sign]; }
+const char *astro_moon_phase_slug(MoonPhaseName phase) { return k_moon_phase_slug[phase]; }
+const char *astro_aspect_slug(AspectType type) { return k_aspect_slug[type]; }