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.

interpreter/src/significance.h

@@ -0,0 +1,57 @@
+#ifndef DECK_SIGNIFICANCE_H
+#define DECK_SIGNIFICANCE_H
+
+/* Header-only dependency on the engine: this module never compiles or
+ * links the engine's own .c files (or the vendored Astronomy Engine),
+ * only shares its struct/enum definitions. That's what makes it testable
+ * with a hand-built DailyReading instead of a real reading_generate()
+ * call - see interpreter/tests/significance_test.c. */
+#include "../../engine/src/reading.h"
+
+#include <stdbool.h>
+
+#define MAX_SIGNIFICANT_ITEMS 5
+
+/* Only aspects compete for the top-5 today. Kept as an explicit tag
+ * (rather than assuming "item == aspect") so a future kind - e.g. a
+ * notable moon phase or a house ingress - can be added without
+ * reshaping this struct. */
+typedef enum {
+  ITEM_ASPECT = 0
+} SignificantItemKind;
+
+typedef struct {
+  SignificantItemKind kind;
+  Aspect aspect; /* valid when kind == ITEM_ASPECT */
+  double score;  /* higher = more significant; no fixed upper bound */
+} SignificantItem;
+
+/* Overall classification of the day, derived from the top item's score.
+ * A small enum, not a raw number, so callers (rendering, i18n, "does
+ * this deserve Celtic Cross framing") don't each have to invent their
+ * own thresholds. */
+typedef enum {
+  DAY_QUIET = 0,
+  DAY_NOTABLE,
+  DAY_SIGNIFICANT,
+  DAY_MAJOR
+} DaySignificance;
+
+typedef struct {
+  SignificantItem top_items[MAX_SIGNIFICANT_ITEMS];
+  int top_item_count; /* <= MAX_SIGNIFICANT_ITEMS; fewer if the day had
+                        * fewer than 5 aspects in orb */
+  DaySignificance day_level;
+} DailyInterpretation;
+
+/* Reads reading->transits.aspects[] only - never touches natal/spread,
+ * and never calls back into the engine, so a hand-built DailyReading is
+ * a complete, valid input. */
+void interpret_daily_reading(const DailyReading *reading, DailyInterpretation *out);
+
+/* True once day_level reaches the threshold at which the Celtic Cross
+ * reading should get a "this matters" framing sentence naming
+ * interp->top_items[0]. */
+bool interpretation_deserves_framing(const DailyInterpretation *interp);
+
+#endif