Consolidate word filtering into shared isJunkWord helper

Replace copy-pasted hasLetter/containsDigit/containsSpace/containsNonLatin
checks across 9 loaders with a single isJunkWord() call. SCOWL and CEDICT
keep their own filtering for valid reasons (SCOWL rejects hyphens/apostrophes,
CEDICT handles non-Latin Chinese characters).

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

internal/loader/scowl.go

@@ -158,3 +158,8 @@ func hasLetter(s string) bool {
 	}
 	return false
 }
+
+// isJunkWord returns true if the word should be rejected from the dictionary.
+func isJunkWord(word string) bool {
+	return word == "" || !hasLetter(word) || containsDigit(word) || containsSpace(word) || containsNonLatin(word)
+}