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/main.c

@@ -0,0 +1,123 @@
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "reading_io.h"
+#include "significance.h"
+
+static void print_usage(const char *prog) {
+  fprintf(stderr,
+    "Usage: %s [reading.json]\n\n"
+    "Reads a DailyReading in deck-engine's --format json shape - from the\n"
+    "given file, or from stdin if no file is given (or it is \"-\") - and\n"
+    "prints a significance report: the day's overall level and the top\n"
+    "aspects driving it.\n\n"
+    "Typical use:\n"
+    "  dist/deck-engine ... --format json | %s\n"
+    "  dist/deck-engine ... --format json > reading.json && %s reading.json\n",
+    prog, prog, prog);
+}
+
+static char *read_all(FILE *f, size_t *out_length) {
+  size_t cap = 4096;
+  size_t len = 0;
+  char *buf = malloc(cap);
+
+  size_t n;
+  while ((n = fread(buf + len, 1, cap - len, f)) > 0) {
+    len += n;
+    if (len == cap) {
+      cap *= 2;
+      buf = realloc(buf, cap);
+    }
+  }
+  buf = realloc(buf, len + 1);
+  buf[len] = '\0';
+  *out_length = len;
+  return buf;
+}
+
+/* Display names for the report - deliberately a small local table rather
+ * than linking engine/src/astro.c's astro_body_name()/astro_aspect_name()
+ * (which would pull in the vendored Astronomy Engine just for cosmetic
+ * text). Same policy as reading_io.c's slug tables. */
+static const char *body_display_name(Body body) {
+  static const char *const names[NUM_BODIES] = {
+    "Sun", "Moon", "Mercury", "Venus", "Mars",
+    "Jupiter", "Saturn", "Uranus", "Neptune", "Pluto",
+  };
+  return names[body];
+}
+
+static const char *aspect_display_name(AspectType type) {
+  static const char *const names[5] = {
+    "Conjunction", "Sextile", "Square", "Trine", "Opposition",
+  };
+  return names[type];
+}
+
+static const char *day_level_name(DaySignificance level) {
+  switch (level) {
+    case DAY_QUIET: return "Quiet";
+    case DAY_NOTABLE: return "Notable";
+    case DAY_SIGNIFICANT: return "Significant";
+    case DAY_MAJOR: return "Major";
+  }
+  return "Unknown";
+}
+
+int main(int argc, char **argv) {
+  if (argc > 2 || (argc == 2 && (strcmp(argv[1], "--help") == 0 || strcmp(argv[1], "-h") == 0))) {
+    print_usage(argv[0]);
+    return argc > 2 ? 1 : 0;
+  }
+
+  FILE *in = stdin;
+  bool opened_file = false;
+  if (argc == 2 && strcmp(argv[1], "-") != 0) {
+    in = fopen(argv[1], "r");
+    if (!in) {
+      fprintf(stderr, "error: could not open %s\n", argv[1]);
+      return 1;
+    }
+    opened_file = true;
+  }
+
+  size_t length;
+  char *text = read_all(in, &length);
+  if (opened_file) fclose(in);
+
+  DailyReading reading;
+  bool loaded = reading_load_json(text, length, &reading);
+  free(text);
+  if (!loaded) {
+    fprintf(stderr, "error: could not parse input as deck-engine --format json output\n");
+    return 1;
+  }
+
+  DailyInterpretation interp;
+  interpret_daily_reading(&reading, &interp);
+
+  printf("Day significance: %s\n", day_level_name(interp.day_level));
+  if (interpretation_deserves_framing(&interp)) {
+    printf("(a major transit today - worth a deeper Celtic Cross look)\n");
+  }
+  printf("\n");
+
+  if (interp.top_item_count == 0) {
+    printf("No notable transits today.\n");
+    return 0;
+  }
+
+  printf("Top %d significant transit%s:\n", interp.top_item_count,
+         interp.top_item_count == 1 ? "" : "s");
+  for (int i = 0; i < interp.top_item_count; i++) {
+    const SignificantItem *item = &interp.top_items[i];
+    printf("  %d. Transiting %s %s natal %s (orb %.1f\xc2\xb0, score %.2f)\n", i + 1,
+           body_display_name(item->aspect.transiting_planet),
+           aspect_display_name(item->aspect.type),
+           body_display_name(item->aspect.natal_planet), item->aspect.orb, item->score);
+  }
+  return 0;
+}