Suppress subtitle annotations for grammar fragments

- Hide annotation metadata for auxiliary inflection and ja-nai endings
- Preserve lexical `くれる` forms and add regression coverage

backlog/tasks/task-311 - Suppress-auxiliary-inflection-fragments-from-subtitle-annotations.md

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+---
+id: TASK-311
+title: Suppress auxiliary inflection fragments from subtitle annotations
+status: Done
+assignee: []
+created_date: '2026-05-02 09:07'
+updated_date: '2026-05-02 09:10'
+labels:
+  - tokenizer
+  - annotations
+  - bug
+dependencies: []
+priority: medium
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+Suppress standalone Japanese auxiliary/inflection subtitle fragments such as `れる` and `れた` from frequency/JLPT/N+1/known annotation styling while keeping lexical verbs such as `くれ` / `くれる` annotatable. Tokens must remain hoverable; only annotation metadata should be stripped.
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+<!-- AC:BEGIN -->
+- [x] #1 `れる` and `れた`-style standalone helper fragments render as plain hoverable subtitle tokens.
+- [x] #2 Lexical verbs like `くれ` / `くれる` remain eligible for annotation.
+- [x] #3 Regression tests cover unit filter behavior and tokenizer integration.
+<!-- AC:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+Implemented with TDD. Added failing coverage first for standalone `れる`/`れた` auxiliary fragments and a lexical `くれ`/`くれる` guard. Updated the shared subtitle annotation filter to strip annotation metadata for kana-only auxiliary inflection fragments identified by MeCab POS (`助動詞` only, or `動詞/接尾` with optional trailing `助動詞`) while preserving lexical `くれ` as `くれる` when tagged `動詞/自立`. Added tokenizer integration coverage for `れた` and neighboring lexical N+1 behavior.
+<!-- SECTION:NOTES:END -->
+
+## Final Summary
+
+<!-- SECTION:FINAL_SUMMARY:BEGIN -->
+Suppressed annotation metadata for standalone auxiliary inflection fragments such as `れる` and `れた` in subtitle tokens, leaving them hoverable but plain. Preserved lexical `くれ` -> `くれる` verb metadata when MeCab tags it as `動詞/自立`.
+
+Added unit and tokenizer regression coverage, plus a release fragment in `changes/311-auxiliary-inflection-annotation-filter.md`.
+
+Validation: targeted annotation/tokenizer tests passed; `bun run typecheck` passed; `bun run changelog:lint` passed. `bun run test:fast` was attempted twice and failed in unrelated `src/core/services/subsync.test.ts` cross-file state (`window.electronAPI` undefined), while `bun test src/core/services/subsync.test.ts` passes by itself.
+<!-- SECTION:FINAL_SUMMARY:END -->