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/affix.go

@@ -48,7 +48,7 @@ func (a AffixLoader) Load(db *sql.DB, dataDir string) error {
 	seen := make(map[string]struct{})
 	for _, entry := range dicEntries {
 		for _, form := range expandWord(entry.word, entry.flags, rules) {
-			if !hasLetter(form) || containsDigit(form) || containsSpace(form) {
+			if isJunkWord(form) {
 				continue
 			}
 			if _, dup := seen[form]; dup {
@@ -194,7 +194,7 @@ func parseDicFile(path string) ([]dicEntry, error) {
 			flags = parts[1]
 		}
 
-		if containsDigit(word) || containsNonLatin(word) {
+		if isJunkWord(word) {
 			continue
 		}