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>

src/ngx_http_ipng_stats_module.c

@@ -66,7 +66,9 @@ typedef struct {
 extern ngx_module_t  ngx_http_log_module;
 
 
-#define NGX_HTTP_IPNG_STATS_VERSION          "0.1.0"
+/* NGX_HTTP_IPNG_STATS_VERSION is generated at build time from the
+ * top-level Makefile's VERSION variable — see `make version-header`. */
+#include "version.h"
 #define NGX_HTTP_IPNG_STATS_SCHEMA_VERSION   2
 
 /* Default histogram buckets in milliseconds (FR-2.3). */