The union was discriminated with a `cause._tag` ternary inside `Effect.mapError`
even though both members are statically known tagged errors; `Effect.catchTags`
says the same thing without the manual check. The comment above `read` claiming
both carry a `reason` was stale — Firefox's no longer does.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>