deb packaging: build-deb.sh + packaging/deb, icons from ico/

- scripts/build-deb.sh: binary built in ubuntu:22.04 (glibc 2.35 baseline),
  Depends via dpkg-shlibdeps, DEB_REVISION (0.1.28-1), changelog.Debian.gz,
  postinst/postrm cache refresh, REPO=1 local apt index, self-checks
  (version/upgrade path, stale binary, desktop-file-validate)
- packaging/deb/: control.in, rcalc.desktop, appdata metainfo, changelog, hooks
- ico/: new app icon in 32/48/64/96/256/512, used for the hicolor slots
- convert.rs: fix clippy::approx_constant error in roundtrip test (PI constant)
- README: install/update/release instructions

packaging/deb/postinst

@@ -0,0 +1,31 @@
+#!/bin/sh
+# Maintainer script of the generated rcalc .deb (see scripts/build-deb.sh).
+set -e
+
+case "$1" in
+    configure)
+        ;;
+    abort-upgrade|abort-remove|abort-deconfigure)
+        exit 0
+        ;;
+    *)
+        echo "postinst called with unknown argument \`$1'" >&2
+        exit 1
+        ;;
+esac
+
+# Refresh the XDG caches. Never fail the installation because of them: the
+# tools may be absent (minimal systems, chroot, dpkg -x in a container).
+if command -v update-desktop-database >/dev/null 2>&1; then
+    update-desktop-database -q /usr/share/applications || true
+fi
+
+if command -v gtk-update-icon-cache >/dev/null 2>&1; then
+    gtk-update-icon-cache -q -t -f /usr/share/icons/hicolor || true
+fi
+
+if command -v appstreamcli >/dev/null 2>&1; then
+    appstreamcli refresh --force >/dev/null 2>&1 || true
+fi
+
+exit 0