fix: suppress N+1 for kana-only candidates and fix minSentenceWords coun

- Treat kana-only tokens with surrounding subtitle punctuation (…, ―, etc.) as kana-only so they are not promoted to N+1 targets
- Exclude unknown tokens filtered from N+1 targeting from the minSentenceWords count so filtered kana-only unknowns cannot satisfy sentence length threshold
- Add regression tests for kana-only candidate suppression and filtered-unknown padding cases

backlog/tasks/task-310 - Suppress-N1-highlight-for-kana-only-candidate-sentences.md

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+---
+id: TASK-310
+title: Suppress N+1 highlight for kana-only candidate sentences
+status: Done
+assignee:
+  - Codex
+created_date: '2026-04-28 06:55'
+updated_date: '2026-04-28 07:04'
+labels:
+  - tokenizer
+  - n+1
+dependencies: []
+priority: medium
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Reduce noisy N+1 subtitle annotations when the only unknown candidates in a sentence are kana-only hiragana or katakana words, such as mostly-kana subtitle lines where highlighting a particle/helper-like token is low value.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [x] #1 N+1 annotation does not mark a kana-only unknown target when all N+1 candidates in the sentence are kana-only.
+- [x] #2 N+1 annotation continues to mark kanji or mixed-script unknown targets in otherwise eligible sentences.
+- [x] #3 A focused regression test covers the kana-only candidate case.
+- [x] #4 N+1 minimum sentence word count excludes tokens stripped by the subtitle annotation filter, so filtered grammar/noise tokens cannot satisfy minSentenceWords.
+<!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+1. Keep the existing N+1 target eligibility guard: kana-only subtitle surfaces do not become N+1 targets.
+2. Add a focused regression in src/core/services/tokenizer/annotation-stage.test.ts proving annotation-filtered tokens do not count toward ankiConnect.nPlusOne.minSentenceWords.
+3. Verify the new regression fails before code changes.
+4. Patch src/token-merger.ts so the N+1 minimum sentence word count uses the same subtitle-annotation eligibility filter as annotation rendering, excluding filtered particles/auxiliaries/noise from the count.
+5. Re-run focused tokenizer tests, then update TASK-310 acceptance criteria and final notes.
+<!-- SECTION:PLAN:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+Initial context: current token-merger has an existing surface-level kana-only guard in isNPlusOneCandidateToken, added in commit 9e4ad907. Need decide whether to broaden behavior to lookup/headword forms or verify current behavior only.
+
+Implemented by treating kana-only N+1 candidates as kana-only even when their token surface includes surrounding subtitle punctuation such as ellipsis or dashes. Focused regression was red before the token-merger change: スイッチ… was marked true, then passed after the guard update. test:env initially hit an unrelated immersion-tracker active_days timing/order failure and Bun follow-on loader error; the failing test passed in isolation and the full test:env rerun passed.
+
+Reopened for follow-up scope: minSentenceWords must count annotation-eligible tokens only, not tokens stripped from annotation metadata.
+
+Implemented follow-up minSentenceWords behavior: unknown tokens filtered from N+1 targeting no longer contribute to sentence length; known eligible tokens and true N+1 candidates still count.
+<!-- SECTION:NOTES:END -->
+
+## Final Summary
+
+<!-- SECTION:FINAL_SUMMARY:BEGIN -->
+Changed N+1 sentence-length counting so minSentenceWords only counts known eligible words and actual N+1 target candidates. Unknown tokens filtered from N+1 targeting, including kana-only unknowns, no longer pad a sentence into eligibility. Existing annotation-filtered particles/auxiliaries remain excluded. Added regression coverage for the filtered unknown padding case while preserving kanji/mixed-script target behavior.
+
+Verification: new regression failed before implementation; `bun test src/core/services/tokenizer/annotation-stage.test.ts -t "N\\+1"` pass; full `bun test src/core/services/tokenizer/annotation-stage.test.ts` pass; `bun test src/core/services/tokenizer.test.ts -t "N\\+1"` pass; `bun run typecheck` pass.
+<!-- SECTION:FINAL_SUMMARY:END -->