Rework build system; cut release 1.3.1-1

Replace the dpkg-buildpackage / debhelper rig with the same pattern
used in vpp-maglev: 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 'govpp-snmp-agentx --version' is the source of
truth for which build is running. main.go's Version constant becomes
a var to make this work; the old sync-version make target is gone.

Removes the now-unused debian/{control,rules,postinst,prerm,*.debhelper}
files and adds build/ to .gitignore.

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

debian/changelog

@@ -1,3 +1,28 @@
+govpp-snmp-agentx (1.3.1-1) bookworm; urgency=medium
+
+  * Fix startup race that left every ifSpeed/ifHighSpeed row stuck at
+    1Gbps until restart. The stats routine could rebuild the MIB before
+    the interface-event routine had pushed any details, locking the
+    static fields at their "unknown" default; the rebuild was only
+    re-triggered on interface-set changes, so the wrong values were
+    never corrected. The stats path now pushes the details it already
+    fetches into the MIB, and a dirty flag forces a rebuild on the
+    next poll whenever Speed/MAC/MTU/admin/oper actually change.
+
+ -- Pim van Pelt <pim@ipng.ch>  Tue, 06 May 2026 12:00:00 +0000
+
+govpp-snmp-agentx (1.3.0-1) bookworm; urgency=medium
+
+  * Fix ifSpeed for >2.5Gbps interfaces (10G/25G/100G): the row was
+    silently skipped, which let monitoring stacks fall back to a
+    hard-coded 1Gbps. Always emit ifSpeed and cap at uint32 max per
+    RFC 2863; ifHighSpeed continues to carry the real Mbps value.
+  * Switch the build system to cross-compiled, CGO-free amd64+arm64
+    .deb packages produced by debian/build-deb.sh; inject the version
+    at link time via -ldflags.
+
+ -- Pim van Pelt <pim@ipng.ch>  Tue, 06 May 2026 00:00:00 +0000
+
 govpp-snmp-agentx (1.2.3-1) bookworm; urgency=medium
 
   * Fix VPP stats filtering to exclude deleted interfaces