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/tests/smoke_test.c

@@ -94,6 +94,49 @@ static void test_reading_generate_smoke(void) {
   printf("PASS test_reading_generate_smoke\n");
 }
 
+static void test_reading_print_json_shape(void) {
+  BirthData birth = {
+    .year = 1990, .month = 5, .day = 14,
+    .hour = 14, .minute = 32,
+    .utc_offset_hours = 2.0,
+    .latitude = 52.52, .longitude = 13.405,
+  };
+
+  DailyReading r;
+  reading_generate("smoke-test-seed", &birth, 1751500000, &r);
+
+  FILE *tmp = tmpfile();
+  assert(tmp);
+  reading_print_json(&r, tmp);
+
+  rewind(tmp);
+  char buf[16384];
+  size_t n = fread(buf, 1, sizeof buf - 1, tmp);
+  buf[n] = '\0';
+  fclose(tmp);
+
+  /* Loose structural checks - not a full JSON validator, just enough to
+   * catch a broken serializer (unbalanced braces, a missing section, or
+   * a stray astro_*_name()/tarot_*_name() call leaking translated text
+   * into what must stay language-independent regardless of --lang). */
+  assert(buf[0] == '{');
+  assert(strstr(buf, "\"natal\"") != NULL);
+  assert(strstr(buf, "\"transits\"") != NULL);
+  assert(strstr(buf, "\"spread\"") != NULL);
+  assert(strstr(buf, "\"aspects\"") != NULL);
+  assert(strstr(buf, "\"body\": \"sun\"") != NULL); /* slug, not the display name */
+  assert(strstr(buf, "\"Sun\"") == NULL);            /* never a *_name() value */
+
+  int braces = 0;
+  for (size_t i = 0; i < n; i++) {
+    if (buf[i] == '{') braces++;
+    if (buf[i] == '}') braces--;
+  }
+  assert(braces == 0);
+
+  printf("PASS test_reading_print_json_shape\n");
+}
+
 /* Run with cwd == engine/ (as `make test` does), so the shipped
  * engine/i18n/ .lang files are reachable as i18n/<code>.lang. */
 static void test_i18n_fallback_and_translation(void) {
@@ -130,6 +173,7 @@ int main(void) {
   test_tarot_determinism();
   test_natal_sun_sign();
   test_reading_generate_smoke();
+  test_reading_print_json_shape();
   test_i18n_fallback_and_translation();
   printf("All smoke tests passed.\n");
   return 0;