The bundled Hunspell dictionary (~49k SCOWL roots) lacked common words like
"cybersecurity", "scalable", and "assistantship", and checked words inside
URLs. On macOS, spelling now uses the system NSSpellChecker (same engine as
TextEdit/Safari): far better vocabulary, skips URLs, and respects words the
user has taught macOS. The custom/tech dictionaries and Harper grammar are
unchanged; non-macOS builds keep the Hunspell fallback.
proof_text loaded the dictionary and ran spell/grammar checks
synchronously inside an async command, blocking an async-runtime worker
and holding the engine mutex for the whole call. Move the work to
spawn_blocking so IPC stays responsive under load.