fix: retain frequency rank for honorific prefix-noun tokens

- Add `shouldAllowHonorificPrefixNounFrequency` to exempt お/ご/御 + noun merged tokens from frequency exclusion
- Add regression test for `ご機嫌` asserting rank 5484 is preserved after MeCab enrichment and annotation
- Close TASK-341

backlog/tasks/task-341 - Fix-frequency-highlight-for-honorific-prefix-noun-tokens.md

@@ -0,0 +1,66 @@
+---
+id: TASK-341
+title: Fix frequency highlight for honorific prefix noun tokens
+status: Done
+assignee:
+  - codex
+created_date: '2026-05-05 02:08'
+updated_date: '2026-05-05 02:10'
+labels:
+  - bug
+  - tokenizer
+  - frequency
+dependencies: []
+documentation:
+  - docs/architecture/2026-03-15-renderer-performance-design.md
+priority: high
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+User reported subtitle token `ご機嫌` in `（フランク）ご機嫌が良くないようだな アンドリュー` shows Yomitan/JPDB rank 5484 in popup but is not highlighted as frequent. Frequency annotation currently excludes merged tokens containing default-excluded POS parts such as `接頭詞`; ordinal prefix-noun tokens already have an exception. Desired outcome: honorific prefix + noun lexical tokens like `ご機嫌` keep their valid frequency rank so renderer can apply frequent-token styling, while standalone prefixes and noisy merged grammar fragments remain excluded.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [x] #1 `ご機嫌`-style honorific prefix + noun tokens retain a finite frequency rank after annotation/tokenization when frequency highlighting is enabled.
+- [x] #2 Standalone prefix/noise tokens remain excluded from frequency annotation.
+- [x] #3 Regression test covers the reported `ご機嫌` rank 5484 behavior.
+- [x] #4 Relevant tokenizer/annotation tests pass.
+<!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+1. Add a failing regression around honorific prefix + noun token frequency retention, using `ご機嫌` with rank 5484 and POS `接頭詞|名詞` / `名詞接続|一般`.
+2. Implement a narrow annotation-stage exception for lexical honorific prefix-noun tokens, adjacent to the existing ordinal prefix-noun allowance.
+3. Verify standalone prefix/noise exclusion behavior remains covered.
+4. Run targeted tokenizer/annotation tests and update acceptance criteria/final notes.
+<!-- SECTION:PLAN:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+TDD red verified: `bun test src/core/services/tokenizer.test.ts -t "honorific prefix-noun"` failed with `actual: undefined`, `expected: 5484` before implementation.
+
+Implemented a narrow honorific prefix-noun frequency allowance for merged `お`/`ご`/`御` + noun tokens with POS `接頭詞|名詞` and prefix POS2 `名詞接続`. Existing standalone prefix/noise exclusion tests still pass.
+
+Verification: `bun test src/core/services/tokenizer.test.ts src/core/services/tokenizer/annotation-stage.test.ts` passed (164 tests); `bun run typecheck` passed; `bunx prettier --check src/core/services/tokenizer/annotation-stage.ts src/core/services/tokenizer.test.ts` passed. Repo-wide `bun run format:check:src` still fails on pre-existing `src/core/services/stats-window.ts` formatting.
+<!-- SECTION:NOTES:END -->
+
+## Final Summary
+
+<!-- SECTION:FINAL_SUMMARY:BEGIN -->
+Fixed frequency annotation for lexical honorific prefix-noun tokens such as `ご機嫌`. The annotation filter now allows merged `お`/`ご`/`御` prefix + noun tokens with MeCab POS `接頭詞|名詞` / `名詞接続|...` to retain a valid frequency rank, while standalone prefixes and existing noise filters remain excluded.
+
+Added a tokenizer regression for the reported `ご機嫌` case asserting rank `5484` is preserved after MeCab enrichment and annotation.
+
+Verification:
+- `bun test src/core/services/tokenizer.test.ts -t "honorific prefix-noun"` failed before the fix with `undefined` vs `5484`, then passed after the fix.
+- `bun test src/core/services/tokenizer.test.ts src/core/services/tokenizer/annotation-stage.test.ts` passed (164 tests).
+- `bun run typecheck` passed.
+- `bunx prettier --check src/core/services/tokenizer/annotation-stage.ts src/core/services/tokenizer.test.ts` passed.
+
+Note: repo-wide `bun run format:check:src` currently fails on unrelated existing formatting in `src/core/services/stats-window.ts`.
+<!-- SECTION:FINAL_SUMMARY:END -->

src/core/services/tokenizer.test.ts

@@ -4140,6 +4140,96 @@ test('tokenizeSubtitle keeps frequency for ordinal prefix-noun tokens', async ()
   assert.equal(result.tokens?.[0]?.frequencyRank, 1820);
 });
 
+test('tokenizeSubtitle keeps frequency for honorific prefix-noun tokens', async () => {
+  const result = await tokenizeSubtitle(
+    'ご機嫌が良くない',
+    makeDepsFromYomitanTokens(
+      [
+        { surface: 'ご機嫌', reading: 'ごきげん', headword: 'ご機嫌' },
+        { surface: 'が', reading: 'が', headword: 'が' },
+        { surface: '良くない', reading: 'よくない', headword: '良い' },
+      ],
+      {
+        getFrequencyDictionaryEnabled: () => true,
+        getFrequencyRank: (text) => (text === 'ご機嫌' ? 5484 : null),
+        tokenizeWithMecab: async () => [
+          {
+            headword: 'ご',
+            surface: 'ご',
+            reading: 'ゴ',
+            startPos: 0,
+            endPos: 1,
+            partOfSpeech: PartOfSpeech.other,
+            pos1: '接頭詞',
+            pos2: '名詞接続',
+            isMerged: false,
+            isKnown: false,
+            isNPlusOneTarget: false,
+          },
+          {
+            headword: '機嫌',
+            surface: '機嫌',
+            reading: 'キゲン',
+            startPos: 1,
+            endPos: 3,
+            partOfSpeech: PartOfSpeech.noun,
+            pos1: '名詞',
+            pos2: '一般',
+            isMerged: false,
+            isKnown: false,
+            isNPlusOneTarget: false,
+          },
+          {
+            headword: 'が',
+            surface: 'が',
+            reading: 'ガ',
+            startPos: 3,
+            endPos: 4,
+            partOfSpeech: PartOfSpeech.particle,
+            pos1: '助詞',
+            pos2: '格助詞',
+            isMerged: false,
+            isKnown: false,
+            isNPlusOneTarget: false,
+          },
+          {
+            headword: '良い',
+            surface: '良く',
+            reading: 'ヨク',
+            startPos: 4,
+            endPos: 6,
+            partOfSpeech: PartOfSpeech.i_adjective,
+            pos1: '形容詞',
+            pos2: '自立',
+            isMerged: false,
+            isKnown: false,
+            isNPlusOneTarget: false,
+          },
+          {
+            headword: 'ない',
+            surface: 'ない',
+            reading: 'ナイ',
+            startPos: 6,
+            endPos: 8,
+            partOfSpeech: PartOfSpeech.bound_auxiliary,
+            pos1: '助動詞',
+            pos2: '*',
+            isMerged: false,
+            isKnown: false,
+            isNPlusOneTarget: false,
+          },
+        ],
+        getMinSentenceWordsForNPlusOne: () => 1,
+      },
+    ),
+  );
+
+  assert.equal(result.tokens?.[0]?.surface, 'ご機嫌');
+  assert.equal(result.tokens?.[0]?.pos1, '接頭詞|名詞');
+  assert.equal(result.tokens?.[0]?.pos2, '名詞接続|一般');
+  assert.equal(result.tokens?.[0]?.frequencyRank, 5484);
+});
+
 test('tokenizeSubtitle clears all annotations for explanatory contrast endings', async () => {
   const result = await tokenizeSubtitle(
     '最近辛いものが続いとるんですけど',

src/core/services/tokenizer/annotation-stage.ts

@@ -167,6 +167,27 @@ function shouldAllowOrdinalPrefixNounFrequency(token: MergedToken): boolean {
   );
 }
 
+function shouldAllowHonorificPrefixNounFrequency(token: MergedToken): boolean {
+  const normalizedSurface = token.surface.trim();
+  const normalizedHeadword = token.headword.trim();
+  if (
+    !['お', 'ご', '御'].some(
+      (prefix) => normalizedSurface.startsWith(prefix) || normalizedHeadword.startsWith(prefix),
+    )
+  ) {
+    return false;
+  }
+
+  const pos1Parts = splitNormalizedTagParts(normalizePos1Tag(token.pos1));
+  const pos2Parts = splitNormalizedTagParts(normalizePos2Tag(token.pos2));
+  return (
+    pos1Parts.length >= 2 &&
+    pos1Parts[0] === '接頭詞' &&
+    pos1Parts.slice(1).some((part) => part === '名詞') &&
+    pos2Parts[0] === '名詞接続'
+  );
+}
+
 function isFrequencyExcludedByPos(
   token: MergedToken,
   pos1Exclusions: ReadonlySet<string>,
@@ -187,11 +208,13 @@ function isFrequencyExcludedByPos(
     pos2Exclusions,
   );
   const allowOrdinalPrefixNounToken = shouldAllowOrdinalPrefixNounFrequency(token);
+  const allowHonorificPrefixNounToken = shouldAllowHonorificPrefixNounFrequency(token);
 
   if (
     isExcludedByTagSet(normalizedPos1, pos1Exclusions) &&
     !allowContentLedMergedToken &&
-    !allowOrdinalPrefixNounToken
+    !allowOrdinalPrefixNounToken &&
+    !allowHonorificPrefixNounToken
   ) {
     return true;
   }
@@ -199,7 +222,8 @@ function isFrequencyExcludedByPos(
   if (
     isExcludedByTagSet(normalizedPos2, pos2Exclusions) &&
     !allowContentLedMergedToken &&
-    !allowOrdinalPrefixNounToken
+    !allowOrdinalPrefixNounToken &&
+    !allowHonorificPrefixNounToken
   ) {
     return true;
   }