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>

.gitignore

@@ -3,6 +3,7 @@ build/
 objs/
 *.so
 *.o
+src/version.h
 
 # Debian build outputs (dpkg-buildpackage writes these to the parent dir,
 # but if a helper extracts them here, ignore them)