Add core engine: tarot + astrology reading generator with CLI and i18n

Plain-C, dependency-free engine that produces a personalized daily
Celtic Cross tarot spread and natal-chart/transit astrology reading,
plus a standalone CLI (dist/deck-engine) and run.sh wrapper. Includes
English/German output via a simple key=value translation format, and
docs/diagrams describing the architecture and I/O.

engine/src/reading.h

@@ -0,0 +1,26 @@
+#ifndef DECK_READING_H
+#define DECK_READING_H
+
+#include <stdio.h>
+#include <time.h>
+
+#include "astro.h"
+#include "tarot.h"
+
+typedef struct {
+  NatalChart natal;
+  DailyTransits transits;
+  CelticCrossSpread spread;
+} DailyReading;
+
+/* The real API: this is the only function the watchapp needs to call.
+ * It walks the returned struct directly to lay out its own screens. */
+void reading_generate(const char *tarot_seed, const BirthData *birth,
+                       time_t utc_moment, DailyReading *out);
+
+/* Desktop-only output formats for inspecting a reading before any watch
+ * UI exists. Not used by (and not needed by) the watchapp. */
+void reading_print_text(const DailyReading *r, FILE *out);
+void reading_print_html(const DailyReading *r, FILE *out);
+
+#endif