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

src/convert.rs

@@ -858,7 +858,7 @@ mod tests {
 
     #[test]
     fn roundtrip_all_endians() {
-        let samples = [0.0f32, -1.0, 3.1415927, 12345.678, f32::MIN_POSITIVE];
+        let samples = [0.0f32, -1.0, std::f32::consts::PI, 12345.678, f32::MIN_POSITIVE];
         for e in Endian::all() {
             for &f in &samples {
                 let (w0, w1) = float_to_words(f, e);