161 diagnostic never shows injected source

~[comptime
~tor hello {}
// The trigger is deliberately on LINE 1 with all prose below it: this pin's
// subject is what the reporter renders when a location walks BACKWARD off the
// top of the file, so nothing may precede it. The prose also avoids spelling
// the injected prologue line verbatim, because a file containing that text
// suppresses the source excerpt and would hollow out the assertion below.
//
// Pins: a diagnostic renders the USER'S source, never the compiler's own. The
// std/compiler import is auto-injected ahead of every file; it occupies no
// line the programmer can see or edit, and no caret may ever land on it.
//
// The guarded shape is two invariants at once — the reported line must be a
// real line of this file, and the rendered excerpt must be the user's text.
// A location that underflows into the injected prologue satisfies neither,
// and hands the programmer a line number that does not exist alongside code
// they never wrote.
//
// Sibling pin: 210_160, the same off-by-one where the file is long enough
// that it lands on a real (but wrong) line instead of underflowing.