Red tests for already-written fixes: build the test against mutated header copies

When a reviewer asks for red/green evidence for a fix that is already in a header-only library, write a mutants script instead of rewriting history. For each gate, copy the header, undo one specific line with an exact-match replace that asserts the pattern occurs exactly once, compile the test against the copy, and expect that gate's FAIL line (or a named crash message). The unmutated build must pass in the same script. Gotcha: if the mutated header is included by another header in the original directory via a quoted include, the compiler finds the original first and you get duplicate definitions. Copy the whole include directory into the mutant dir and put that first on the include path. Another gotcha: if two checks share the same fixture, a mutant may be caught by the wrong check (here a hash check also caught a removed type check). Build each hostile input so that only the property under test is wrong.