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/json.h

@@ -0,0 +1,55 @@
+#ifndef DECK_JSON_H
+#define DECK_JSON_H
+
+#include <stdbool.h>
+#include <stddef.h>
+
+/* Minimal, hand-rolled JSON reader - just enough to parse deck-engine's
+ * own --format json output (engine/src/reading.c's reading_print_json).
+ * Not a general-purpose/validating JSON library, and unlike the core
+ * engine this is desktop-only (uses malloc) - the interpreter binary was
+ * never meant to run on the watch itself. */
+
+typedef enum {
+  JSON_NULL,
+  JSON_BOOL,
+  JSON_NUMBER,
+  JSON_STRING,
+  JSON_ARRAY,
+  JSON_OBJECT
+} JsonType;
+
+typedef struct JsonValue JsonValue;
+
+struct JsonValue {
+  JsonType type;
+  union {
+    bool boolean;
+    double number;
+    char *string;
+    struct { JsonValue **items; int count; } array;
+    struct { char **keys; JsonValue **values; int count; } object;
+  } as;
+};
+
+/* Parses `text` into a JsonValue tree; `text` must be null-terminated at
+ * text[length] (every caller here reads a whole file/stream into a
+ * malloc'd buffer and NUL-terminates it before calling this). Returns
+ * NULL on malformed input. On success the caller owns the result and
+ * must free it with json_free(). */
+JsonValue *json_parse(const char *text, size_t length);
+void json_free(JsonValue *value);
+
+/* NULL if `key` isn't present or `value` isn't an object. */
+const JsonValue *json_object_get(const JsonValue *value, const char *key);
+/* 0 if `value` isn't an array. */
+int json_array_count(const JsonValue *value);
+/* NULL if `value` isn't an array or `index` is out of range. */
+const JsonValue *json_array_get(const JsonValue *value, int index);
+/* 0.0 / "" if `value` is NULL or the wrong type - callers that need to
+ * distinguish "missing" from "present but zero" should check
+ * json_object_get()'s/json_array_get()'s NULL return first. */
+double json_as_number(const JsonValue *value);
+const char *json_as_string(const JsonValue *value);
+
+#endif