Firefox has one failure mode — its plaintext database would not open — so a
single-value `reason` literal encoded the same thing as the tag, and `cause`
was optional though every construction site wraps a real failure. The error now
carries the database path and a required cause, and `BrowserImport` supplies
the user-facing reason where it maps the union.

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