Module error-report

ErrorReport — a pretty-printable error-chain formatter.

The record carries a Str message, an optional cause (Option ErrorReport), and a (Vec Str) backtrace placeholder. .pretty renders the chain into a multi-line Str. A Display impl lands once the FmtSpec-aware format pipeline widens.

Construction goes through the free fns error-report and error-report-with-cause, both of which take a pre-formatted message Str; the existential Error-trait constructor arrives later. Backtrace frames will tighten from Str to a structured Frame type alongside the panic-payload rework.