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/Makefile

@@ -0,0 +1,54 @@
+CC      ?= cc
+CFLAGS  ?= -std=c99 -Wall -Wextra -O2
+
+SRC_DIR   := src
+TEST_DIR  := tests
+BUILD_DIR := build
+OBJ_DIR   := $(BUILD_DIR)/obj
+
+# Shares deck-engine's build output directory (see ../engine/Makefile),
+# so a daily run can find both binaries in one place:
+#   dist/deck-engine ... --format json | dist/interpreter-cli
+DIST_DIR := ../dist
+BINARY   := $(DIST_DIR)/interpreter-cli
+
+# Only ever compiles this module's own sources - significance.h/
+# reading_io.h reach into engine/src/ for struct/enum *definitions*
+# (plain #includes), but no engine/src/*.c (or the vendored Astronomy
+# Engine) is compiled or linked here. That's what keeps this module (and
+# its tests) independent of the engine's actual ephemeris/tarot-draw
+# implementation - see significance.c's and reading_io.c's own comments
+# for why each duplicates a small table instead of linking astro.c.
+LIB_SRCS := $(SRC_DIR)/significance.c $(SRC_DIR)/json.c $(SRC_DIR)/reading_io.c
+LIB_OBJS := $(patsubst %.c,$(OBJ_DIR)/%.o,$(LIB_SRCS))
+MAIN_OBJ := $(OBJ_DIR)/$(SRC_DIR)/main.o
+
+SIGNIFICANCE_TEST_OBJ := $(OBJ_DIR)/$(TEST_DIR)/significance_test.o
+JSON_TEST_OBJ         := $(OBJ_DIR)/$(TEST_DIR)/json_test.o
+SIGNIFICANCE_TEST_BIN := $(BUILD_DIR)/significance_test
+JSON_TEST_BIN         := $(BUILD_DIR)/json_test
+
+.PHONY: all test clean
+
+all: $(BINARY)
+
+$(BINARY): $(LIB_OBJS) $(MAIN_OBJ)
+	@mkdir -p $(DIST_DIR)
+	$(CC) $(CFLAGS) -o $@ $^
+
+test: $(SIGNIFICANCE_TEST_BIN) $(JSON_TEST_BIN)
+	./$(SIGNIFICANCE_TEST_BIN)
+	./$(JSON_TEST_BIN)
+
+$(SIGNIFICANCE_TEST_BIN): $(OBJ_DIR)/$(SRC_DIR)/significance.o $(SIGNIFICANCE_TEST_OBJ)
+	$(CC) $(CFLAGS) -o $@ $^
+
+$(JSON_TEST_BIN): $(OBJ_DIR)/$(SRC_DIR)/json.o $(OBJ_DIR)/$(SRC_DIR)/reading_io.o $(JSON_TEST_OBJ)
+	$(CC) $(CFLAGS) -o $@ $^
+
+$(OBJ_DIR)/%.o: %.c
+	@mkdir -p $(dir $@)
+	$(CC) $(CFLAGS) -c -o $@ $<
+
+clean:
+	rm -rf $(BUILD_DIR) $(BINARY)