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/tarot_data.c

@@ -259,3 +259,6 @@ const char *tarot_position_name(CelticCrossPosition position) {
 const char *tarot_position_description(CelticCrossPosition position) {
   return position_field(k_position_slug[position], "desc", k_positions[position].description);
 }
+
+const char *tarot_card_slug(TarotCard card) { return k_card_slug[card]; }
+const char *tarot_position_slug(CelticCrossPosition position) { return k_position_slug[position]; }