docs: fix five accuracy findings in the libadwaita and icon documentation

- CLAUDE.md: the libadwaita dependency paragraph now names both AdwDialog
  widgets (AdwShortcutsDialog and AdwAboutDialog), explains the shared
  on_dialog_closed() handler keyed by pointer-to-pointer, and covers
  both show_about() and show_shortcuts() in the AdwDialog sentence.
- CLAUDE.md: the icon-authoring invariant's explanation of how Icon= in
  the desktop entry resolves now names the installed copies under
  $XDG_DATA_HOME/icons/hicolor/scalable/apps and .../symbolic/apps,
  not the GResource (a desktop shell cannot read a compiled-in resource).
- CLAUDE.md: [help] probe example byte count corrected from 1381 to 1379
  (measured: wc -c src/help/01-overview.txt = 1379).
- README.md + udpscope.gresource.xml: "file, line and column" corrected
  to "file and line number (plus a caret under the offending character)"
  to match xmllint's actual output; also removes -- from the XML comment
  where it was illegal.
- README.md: desktop-file-validate described as a hard failure "when the
  tool is on PATH", matching cmake/install_desktop_entry.cmake lines 44-54.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/icons/udpscope.gresource.xml

@@ -12,8 +12,8 @@
   such a file outright, but the icon *name* still resolves, so `gtk_icon_theme_has_icon()`
   returns true and the icon simply draws nothing. No runtime GTK call can tell the two apart
   (`lookup_icon()` never returns NULL and always reports the size it was asked for), so the
-  check belongs here, where a double hyphen inside a comment fails the build with a file,
-  line and column instead.
+  check belongs here, where a malformed SVG fails the build with a file and line number
+  (plus a caret under the offending character) instead.
 -->
 <gresources>
   <gresource prefix="/org/udpscope/icons">