Rework build system

Replace the dpkg-buildpackage / debhelper rig with the same pattern
used in vpp-maglev and govpp-snmp-agentx: a Makefile that
cross-compiles CGO-free static binaries for amd64 and arm64, plus a
debian/build-deb.sh that stages the .deb directly with dpkg-deb. The
two arch packages drop into build/ and run on any glibc/musl Linux of
the matching arch.

VERSION is parsed once from debian/changelog and injected at link
time via -ldflags "-X 'main.Version=...' -X 'main.Commit=...' -X
'main.Date=...'", so 's3-genindex -version' is the source of truth
for which build is running. The Version constant in main.go becomes
a var to make this work; the old sync-version make target is gone.

Removes the now-unused debian/{control,rules,install,compat} files
and the matching debhelper artifact entries from .gitignore. build/
is added to .gitignore.

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

.gitignore

@@ -1,15 +1,6 @@
 /s3-genindex
+build/
 
 # Generated index files
 **/index.html
 coverage.*
-
-# Debian packaging artifacts
-debian/.debhelper/
-debian/.gocache/
-debian/go/
-debian/s3-genindex/
-debian/files
-debian/*.substvars
-debian/debhelper-build-stamp
-debian/*.debhelper