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>

README.md

@@ -287,7 +287,7 @@ the four action icons are: udpscope is run straight out of its build tree and ha
 The one consequence worth knowing is that a **malformed** SVG cannot be caught at runtime -- the
 icon *name* still resolves, so GTK reports the icon as present and draws nothing -- which is why
 every `<file>` in the manifest carries `preprocess="xml-stripblanks"` and the build fails with a
-file, line and column instead.
+file and line number (plus a caret under the offending character) instead.
 
 **The About dialog is an `AdwAboutDialog`** rather than a `GtkAboutDialog`, and that migration is
 what the app has an icon *for*: the dialog shows it in its own header, which `GtkAboutDialog`
@@ -313,7 +313,7 @@ cmake --build build --target desktop-entry
 ```
 
 That writes `org.udpscope.app.desktop` and both app icons under `$XDG_DATA_HOME` (default
-`~/.local/share`), validates the entry with `desktop-file-validate` as a hard failure, refreshes
+`~/.local/share`), validates the entry with `desktop-file-validate` as a hard failure when the tool is on `PATH`, refreshes
 `update-desktop-database` and `gtk4-update-icon-cache` if they are on `PATH`, and prints the three
 paths it wrote. Those three lines **are** the uninstall instructions: there is no uninstall
 target, because three `rm`s the build already told you about are less to maintain than a target