fix: exclude kana-only n+1 targets

backlog/tasks/task-307 - Exclude-kana-only-words-from-N1-subtitle-targets.md

@@ -0,0 +1,58 @@
+---
+id: TASK-307
+title: Exclude kana-only words from N+1 subtitle targets
+status: Done
+assignee:
+  - codex
+created_date: '2026-04-27 01:52'
+updated_date: '2026-04-27 01:57'
+labels:
+  - tokenizer
+  - annotations
+dependencies: []
+priority: medium
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Subtitle N+1 annotation is over-targeting kana-only or hiragana/katakana tokens that collapse to dictionary words. Adjust targeting so kana-only tokens are not selected as N+1 candidates, while preserving tokenization/hover behavior and other annotation metadata where existing filters allow it.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [x] #1 Kana-only subtitle tokens are not marked as N+1 targets.
+- [x] #2 Kanji or mixed lexical tokens can still be marked as N+1 targets when they are the single unknown candidate in a sentence.
+- [x] #3 Regression coverage demonstrates the kana-only N+1 exclusion.
+<!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+1. Add a failing regression in `src/core/services/tokenizer.test.ts` showing a kana-only Yomitan token is not selected as the single N+1 target, while a mixed lexical token in the same style still can be targeted.
+2. Implement the smallest filter in `src/token-merger.ts`: N+1 candidate selection rejects tokens whose surface is entirely kana; word-count behavior remains governed by existing annotation/POS filters.
+3. Run the focused tokenizer tests, then update task acceptance criteria/final summary.
+<!-- SECTION:PLAN:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+Implemented a surface-level kana-only guard in N+1 candidate selection. Kept existing word-count/POS filtering behavior intact; updated tokenizer and annotation-stage expectations where old tests intentionally allowed kana-only N+1 targets.
+<!-- SECTION:NOTES:END -->
+
+## Final Summary
+
+<!-- SECTION:FINAL_SUMMARY:BEGIN -->
+Summary:
+- Added kana-only surface detection to `isNPlusOneCandidateToken` so hiragana/katakana-only subtitle tokens are not selected as N+1 targets.
+- Added/updated tokenizer and annotation-stage regressions for kana-only targets while preserving non-kana N+1 behavior.
+- Added changelog fragment `changes/307-kana-nplusone-targets.md`.
+
+Verification:
+- `bun test src/core/services/tokenizer.test.ts --test-name-pattern "kana-only N\+1"` failed before the fix with `true !== false`.
+- `bun test src/core/services/tokenizer/annotation-stage.test.ts src/core/services/tokenizer.test.ts` passed.
+- `bun run typecheck` passed.
+- `bun run test:fast` passed.
+- `bun run changelog:lint` passed.
+- `bunx prettier --check src/core/services/tokenizer.test.ts src/core/services/tokenizer/annotation-stage.test.ts src/token-merger.ts changes/307-kana-nplusone-targets.md` passed.
+<!-- SECTION:FINAL_SUMMARY:END -->