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

.gitignore

@@ -1,8 +1,13 @@
 /target
 /target-appimage
+/target-deb
 /.cache
 /dist/*.AppImage
 /dist/rcalc.AppDir/
+/dist/deb-root/
+/dist/*.deb
+/dist/Packages
+/dist/Packages.gz
 /dist/.glibc-build.txt
 .rcalc_last_bump
 /target-windows