build: add Makefile wrapping cmake/ctest workflow

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Makefile

@@ -0,0 +1,24 @@
+BUILD_DIR ?= build
+BIN := $(BUILD_DIR)/src/pipediagram
+
+.PHONY: all configure build run sample test clean
+
+all: build
+
+configure:
+	cmake -S . -B $(BUILD_DIR) -G Ninja
+
+build: configure
+	cmake --build $(BUILD_DIR)
+
+run: build
+	./$(BIN)
+
+sample: build
+	./$(BIN) --sample
+
+test: build
+	ctest --test-dir $(BUILD_DIR) --output-on-failure
+
+clean:
+	rm -rf $(BUILD_DIR)