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/control.in

@@ -0,0 +1,23 @@
+Package: rcalc
+Version: @VERSION@
+Architecture: @ARCHITECTURE@
+Maintainer: @MAINTAINER@
+Section: math
+Priority: optional
+Installed-Size: @INSTALLED_SIZE@
+Depends: @DEPENDS@
+Recommends: libegl1, libgl1, libx11-6, libwayland-client0
+Description: desktop calculator with programmer, engineering and convert tools
+ Calculator inspired by the Windows Calculator (standard mode), written in
+ Rust with the Slint UI toolkit. Modes: Standard, Engineering, Programmer,
+ Convert, CRC and Settings.
+ .
+ Programmer mode shows HEX / DEC / OCT / BIN at once, with QWORD / DWORD /
+ DINT / WORD / INT / BYTE word sizes, bitwise operators and clickable bits.
+ .
+ Convert mode maps a float32 to two 16-bit PLC words in every byte order
+ (AB CD / CD AB / BA DC / DC BA), approximates electronic gear ratios as
+ num/den and converts Char / Hex / Dec ASCII codes.
+ .
+ CRC mode computes Modbus RTU CRC-16 and Modbus ASCII LRC from a string of
+ hex bytes, e.g. "01 03 00 00 00 0A".