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/rcalc.desktop

@@ -0,0 +1,13 @@
+[Desktop Entry]
+Type=Application
+Name=Rcalc
+GenericName=Calculator
+Comment=Calculator with Programmer, Engineering, Convert and CRC tools
+Exec=rcalc
+Icon=rcalc
+Terminal=false
+Categories=Utility;Calculator;
+Keywords=calculator;programmer;hex;bitwise;modbus;crc;lrc;
+StartupNotify=true
+# Wayland app_id / X11 WM_CLASS — must match set_xdg_app_id("rcalc") in src/main.rs
+StartupWMClass=rcalc