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/reading.h

@@ -23,4 +23,10 @@ void reading_generate(const char *tarot_seed, const BirthData *birth,
 void reading_print_text(const DailyReading *r, FILE *out);
 void reading_print_html(const DailyReading *r, FILE *out);
 
+/* Machine-readable serialization of the full struct, using the
+ * language-independent *_slug() accessors (never astro_*_name()/
+ * tarot_*_name()) so the output is identical regardless of --lang. This
+ * is the interchange format interpreter/ reads - see its json.c. */
+void reading_print_json(const DailyReading *r, FILE *out);
+
 #endif