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>

debian/changelog

@@ -1,3 +1,23 @@
+nginx-ipng-stats-plugin (0.2.0-1) unstable; urgency=medium
+
+  * Reduce scrape cardinality.
+    - Status codes are collapsed to six class lanes
+      (1xx/2xx/3xx/4xx/5xx/unknown) on the counter key. Per-(source, vip)
+      counter cardinality is now bounded at six regardless of response mix.
+    - Histograms drop the code label entirely and aggregate across classes
+      per (source, vip).
+    - New nginx_ipng_latency_total counter (with code class label) exposes
+      per-class mean latency via latency_total / requests_total.
+    - New nginx_ipng_bytes_in and nginx_ipng_bytes_out histograms; bucket
+      bounds configurable via the new ipng_stats_byte_buckets directive.
+    - JSON schema bumped to 2. Records are now per-(source, vip) with an
+      inner `classes` map.
+    - Operators needing per-three-digit-code breakdowns should consume
+      ipng_stats_logtail off-host; the stats zone intentionally trades
+      that resolution for a smaller, bounded scrape.
+
+ -- Pim van Pelt <pim@ipng.ch>  Fri, 17 Apr 2026 12:36:57 +0000
+
 nginx-ipng-stats-plugin (0.1.0-1) unstable; urgency=medium
 
   * Initial release.