fix(spellcheck): Linux wordlist flags unknown words; only hunspell counts as CLI

- check_word returned "valid" for a word missing from a loaded wordlist
  (!words.is_empty() && !contains, with contains already false), so the
  wordlist backend never flagged anything.
- aspell on PATH counted as a CLI speller, but only hunspell is invoked,
  so on a machine with aspell and no hunspell every CLI check silently
  failed.
- A bare wordlist offers no suggestions; the test no longer demands
  them from the linux_system backend. Unused PathBuf import dropped.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/control/p1_tests.rs

@@ -819,7 +819,9 @@ fn text_audit_system_spellcheck_flags_misspelled_words() {
             flagged
         );
 
-        // At least one flagged word should have suggestions
+        // At least one flagged word should have suggestions — except from a
+        // bare Linux wordlist, which can tell a word is unknown but has no
+        // way to suggest one without the hunspell CLI.
         let has_suggestions = words.iter().any(|w| {
             w["suggestions"]
                 .as_array()
@@ -827,7 +829,7 @@ fn text_audit_system_spellcheck_flags_misspelled_words() {
                 .unwrap_or(false)
         });
         assert!(
-            has_suggestions,
+            has_suggestions || sp["backend"] == "linux_system",
             "expected at least one word to have suggestions"
         );