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.h

@@ -104,4 +104,13 @@ const char *astro_sign_name(ZodiacSign sign);
 const char *astro_moon_phase_name(MoonPhaseName phase);
 const char *astro_aspect_name(AspectType type);
 
+/* Stable, language-independent identifiers (e.g. "sun", "waxing_crescent",
+ * "square") - the same strings i18n.c's lookup keys are built from. Used
+ * for machine-readable output (reading_print_json) that must stay
+ * identical regardless of --lang, unlike astro_*_name() above. */
+const char *astro_body_slug(Body body);
+const char *astro_sign_slug(ZodiacSign sign);
+const char *astro_moon_phase_slug(MoonPhaseName phase);
+const char *astro_aspect_slug(AspectType type);
+
 #endif