chore(naming): Make the public API a bit easier to reason about. Keep names consistent

[qTipTip]

• add coerce_to_enum to keep string
• use coerce_to_enum in registry
• accept string, but coerce_to_enum immediately
• a -> alpha, freq -> frequency, add opacity as a derived value, add deprecation warnings
• n -> palette_size, num-threads -> max-workers
• update test call-sites after renames
• update smoke-test, and fix it silently not running
• update docs
• update changelog