feat(tokenizer): refine Yomitan grouping and parser tooling

- map segmented Yomitan lines into single logical tokens and improve candidate selection heuristics

- limit frequency lookup to selected token text with POS-based exclusions and add debug logging hook

- add standalone Yomitan parser test script, deterministic utility-script shutdown, and docs/backlog updates

src/types.ts

@@ -56,7 +56,6 @@ export interface MergedToken {
   isNPlusOneTarget: boolean;
   jlptLevel?: JlptLevel;
   frequencyRank?: number;
-  frequencyLookupTerms?: string[];
 }
 
 export type FrequencyDictionaryLookup = (term: string) => number | null;