Status: removed. Measured G4 showed no real speed claim (StringBuffer ~1.08×; Hashtable vs ConcurrentHashMap single-thread weak/negative). Zombie off deleted — no matcher.

Slug never reused.
Historical JSONL/md kept beside this note; fixtures under _removed/fixtures/.
Gate: measured-no-evidence ⇒ cannot remain in active lint.

jp-legacy-synchronized-type — removed 2026-10-22 (Asia/Shanghai)