Release v0.2.0: single-source the version, wildcard it in docs

Introduces a VERSION variable in the top-level Makefile as the
authoritative source for the module's reported version. A new
version-header target writes src/version.h only when the content
would change, so no-op rebuilds don't rewrite the file. The C source
#includes that header in place of a hardcoded #define; the
user-guide's install example is wildcarded
(libnginx-mod-http-ipng-stats_*_amd64.deb) so it doesn't drift.

The design doc still references v0.2.0 by name — operators read it as
a point-in-time description, not a moving target.

debian/changelog keeps its own 0.2.0-1 entry because dpkg reads the
package version from there directly; the e2e test is updated to match
the JSON schema bump to 2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Makefile

@@ -18,9 +18,16 @@ MODULE_NAME := ngx_http_ipng_stats_module
 MODULE_DIR  := $(CURDIR)
 BUILD_DIR   := $(CURDIR)/build
 
+# Single source of truth for the module version. When cutting a release,
+# bump this AND add a matching top entry to debian/changelog — dpkg reads
+# the package version from there directly. The C code picks up VERSION
+# via the generated src/version.h (written by the version-header target
+# below and depended on by the module build).
+VERSION     := 0.2.0
+
 NGINX_SRC ?=
 
-.PHONY: help build pkg-deb robot-test install-deps clean fetch-nginx-src
+.PHONY: help build pkg-deb robot-test install-deps clean fetch-nginx-src version-header
 
 TEST ?= tests/
 
@@ -50,7 +57,19 @@ build: $(BUILD_DIR)/$(MODULE_NAME).so
 	@echo "  echo 'load_module modules/$(MODULE_NAME).so;' | sudo tee /etc/nginx/modules-enabled/50-mod-http-ipng-stats.conf"
 	@echo "  sudo nginx -t && sudo nginx -s reload"
 
-$(BUILD_DIR)/$(MODULE_NAME).so: fetch-nginx-src
+# version-header: write src/version.h iff its contents would change. The
+# target is .PHONY so it's re-evaluated every build, but the file itself
+# is only touched on VERSION bumps — keeps the .so from rebuilding when
+# nothing has actually changed.
+version-header:
+	@NEW='#define NGX_HTTP_IPNG_STATS_VERSION "$(VERSION)"'; \
+	if [ ! -f $(MODULE_DIR)/src/version.h ] \
+	   || [ "$$NEW" != "$$(cat $(MODULE_DIR)/src/version.h)" ]; then \
+		echo "Generating src/version.h (VERSION=$(VERSION))"; \
+		echo "$$NEW" > $(MODULE_DIR)/src/version.h; \
+	fi
+
+$(BUILD_DIR)/$(MODULE_NAME).so: version-header fetch-nginx-src
 	@set -e; \
 	if [ -z "$(NGINX_SRC)" ]; then \
 		NGX_SRC="$(BUILD_DIR)/nginx-src"; \
@@ -154,4 +173,5 @@ install-deps:
 
 clean:
 	rm -rf $(BUILD_DIR) tests/.venv tests/out
+	rm -f $(MODULE_DIR)/src/version.h
 	-dh_clean 2>/dev/null || true