106 raw slice return caret locates the tor

// Pins: the raw-slice return rejection points at the `~tor` that carries the
// offending return type. 510_104 pins the MESSAGE for this identical source;
// this pin holds the other half of the same diagnostic — WHERE it lands.
//
// The guarded shape is a diagnostic raised at end-of-declaration: the reporter
// is handed a cursor that has already advanced past the construct, so the
// caret must be walked back to the line the type was written on rather than
// reported one past it, which on a file this short is past EOF entirely.
~tor echo { msg: string } -> []const u8