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.appdata.xml.in

@@ -0,0 +1,35 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- @RELEASE_DATE@ = UTC date of the build; @VERSION@ comes from Cargo.toml. -->
+<component type="desktop-application">
+  <id>rcalc.desktop</id>
+  <metadata_license>CC0-1.0</metadata_license>
+  <!-- LICENSE file is missing in the repository: NOASSERTION until it is added. -->
+  <project_license>NOASSERTION</project_license>
+  <name>Rcalc</name>
+  <summary>Calculator with programmer, engineering, convert and CRC modes</summary>
+  <description>
+    <p>
+      Desktop calculator inspired by the Windows Calculator. Standard mode uses
+      exact rational arithmetic, so 0.1 + 0.2 really gives 0.3.
+    </p>
+    <ul>
+      <li>Standard: + − × ÷, %, ±, 1/x, x², √x, memory, formula panel</li>
+      <li>Engineering: trigonometry, log/pow, DEG/RAD, SI prefixes</li>
+      <li>Programmer: simultaneous HEX / DEC / OCT / BIN, QWORD … BYTE word
+        sizes, bitwise ops, clickable bits</li>
+      <li>Convert: float32 to two 16-bit words (AB CD / CD AB / BA DC / DC BA),
+        gear ratio num/den, ASCII codes</li>
+      <li>CRC: Modbus RTU CRC-16 and Modbus ASCII LRC from hex bytes</li>
+      <li>Full keyboard input, three themes, three button sizes</li>
+    </ul>
+  </description>
+  <categories>
+    <category>Utility</category>
+    <category>Calculator</category>
+    <category>Science</category>
+  </categories>
+  <releases>
+    <release version="@VERSION@" date="@RELEASE_DATE@"/>
+  </releases>
+  <content_rating type="oars-1.1"/>
+</component>